Added precedence unit test for 'Until'.
Unit test for 'Week' keyword.
Implemented 'Week' keyword.
Unit test for 'Month' keyword.
Naive implementation of 'Month'. Checkpoint before implementing concept of 'Next Month'.
Added concept of a Match Index.
Refactored 'This' and 'Next' to use the new match index functionality.

* N.B. Unit tests current pass in isolation, but the parser is currently
* sharing some state. We need to factor this state out of the parser.