the `SelectClause.computeSchema` method aggregates a `resultSchema`
member by appending all of the ColumnInfos for each of the selectValues
in the statement.

However, when efforts were made to make Schema less "mutable", I think
a change was overlooked that causes all but the last selectValue's
ColumnInfos to remain in the resultSchema.

I believe this was left undetected because the resultSchema of the
SelectClause isn't really used in many places, except for nested
subqueries. This problem was found by a student, that the first out of
two columns in a nested subquery would "disappear" from the schema.

A number of bugfixes caused by some of the changes in the arithmetic
type-coercion rules, and the parser-framework changes.

COUNT(DISTINCT a) wasn't being handled correctly due to an issue in
the NanoSQLTranslator flow-control.

The SumAvg/StdDevVar aggregate implementations are now updated to
produce double-precision results when calculating their values.  The
type-converter had been changed to do integer division when the LHS
and RHS are both integers, and this was breaking tests of these
aggregate functions.

The existing SQL tests are now all assigned to the "sql" group, and a
subset are also now assigned to the "hw2" group if they should be
supported by the SimplePlanner.

The Maven pom.xml is also updated to include the "hw2" tests.

This commit includes the HW2 design document and logistics document.

It also includes the InvalidSQLException to throw when a SQL query has
a semantic error.

If the last-modified date for any of the sources is higher than the
last-modified date of the JAR produced by maven, then a warning will be
emitted to make sure the user is aware that her/his changes may not be
in effect.

This modification also fails silently, e.g. if `find` is missing, or if
the user doesn't have the right version of `stat` to support the printf
flag.

NanoDB client now supports shell commands of the form \cmd, which are
handled within the InteractiveClient, not by the server.  There are two
commands:

\help prints out useful help information

\source filename.sql will run all commands contained in the file
filename.sql

There are surely interesting wrinkles to iron out.  For example, if the
sourced file contains an "EXIT" command then it will exit the client.
Also, an error should probably terminate the evaluation of a sourced
file, but currently it does not.

The nanodb.bat file now behaves in the same way that the *nix nanodb
script works, so that NanoDB can be started from the Windows cmd shell.

A number of correctness tests are contained in the TestHeapTableFormat
class.  The tests are activated in the Maven test configuration.

Added the designdoc and logistic info documents in a new directory.

The Buffer Manager concurrency test wasn't closing the data files it
created, so they couldn't be deleted during the cleanup phase on
Windows.  This caused test failures on Windows but not on *nix.

Somehow when upgrading the NanoDB parser to ANTLR4, I forgot to include
the SHOW STATS command.  This is essential for the storage lab, so of
course I added it back in as quickly as possible.

Updated the nanodb script to properly handle being run under Git Bash.
Needed to wrap the Java program with "winpty" when starting it from Git
Bash.

Also updated the Surefire configuration to not require all tests to pass
before building a JAR file.  There are a few tests that aren't working
on Windows due to file-locking differences from *NIX.

Some tests were failing on my box because I've got my locale set to
Portuguese; this shouldn't be an issue, since the NanoDB tests are
explicitly written in English. In order to fix Date parsing language-
agnostically, I just explicitly set the locale to `Locale.ENGLISH`.

The Buffer Manager wasn't tied into the property-registry mechanism,
so it wouldn't notice when the page-cache size was changed.  This is
now hooked together with a property-observer interface.