Add variables to the grammar

This change set also includes a bit of refactoring:

- Renamed jsonlog to opalog
- Renamed Dictionary to Object
- Split AST into separate files
- Tweaked parser definition to separate terms with whitespace
- Renamed helper functions in parser_test.go to distinguish from cases
- Moved reflection helper into test suite and renamed
- Modified Term.String() to make output more readable
- Reorganized the grammar file
- Allow scalars and variables as object keys. We will deal with this when
serializing to JSON.