Fixes #701: Add the OVER clause and include it in the Function tokens.

[r33s3n6]

Resolves #701

• ran the tests (pytest)
• all style issues addressed (flake8)
• your changes are covered by tests
  (N/A) your changes are documented, if needed

Group tokens such as OVER exist_win or OVER (PARTITION BY...) as the OVER clause before grouping Identifier and Function. Extend tokens of Function to include the OVER clause if it is next to the Parenthesis of function parameters.

[andialbrecht]

Thanks a lot!