Big Code Refactor

- Updated code structure to comply with conventional Ruby gem code structure
- Added base classes to handle fetching documents from Google Drive
- Restricted api class to only Google Documents (no spreadsheets, presentations, etc.)
- Added test for all the new classes.
- Setup guard with `rspec` and `rubocop` for the development environment.
- Created cassettes for `vcr` to make test deterministic (don't need to auth with the Google API to run tests anymore).
- Added test coverage trackers.
Is this style for line length concerns? Personally I think `raise SkoogleDocs::Errors::DocumentNotFound unless doc.status == 200` reads a little better but I'm probably wrong.