Add improved expression parsing
Add expression bug fixes

- Corrected serialization issue where expressions were not being saved
  to disk properly.
- Fixed incorrect ordering of arguments in ExpressionParser.
- Operator GetValue() changed to actually return an object of type value
  in anticipation for expressions replacing the functionality of
  RenPyVariable.
Add factory for ExpressionParsers
Add support for Ren'Py while statements. Fixes #23

- Also added support for moving to the previous statement in the
  execution state.