Implementing hashCode for ColumnReference, Aggregate, and Parameter.

Addresses first three classes in Issue #37.

- hashCode is implemented for some of the more major cases.
- Tests are in place to confirm behavior.
Util test code cleanup.

- Added more cases.
- Removed unnecessary assertion.
- Moved the test to a more traditional location.
Merge pull request #41 from dclements/hashCodes

Hash Codes