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.
I think I would break it up like:

```ruby
unless doc.status == 200
  raise SkoogleDocs::Errors::DocumentNotFound
end
```

I actually don't really like doing these kinds of exception cases with the postfix conditional notation (imho it makes it too hard to see when exactly you're throwing an exception) so I tend to put the condition first. Just a minor personal style thing though!