Google Drive integration

**Integration with Google Drive** will allow us to add next functionality:

- Adding files to GD directly from our app
- Downloading files from GD
- Sharing files between users
- **Showing presenations** using Google Docs

This might be really powerful and handy tool for our purposes. Even more handly than Amazon S3. 


**Before start:**

- Complete this free course from CodeSchool. https://www.codeschool.com/courses/discover-drive
- Find gem for easier integration. I suppose we can use this one: https://github.com/gimite/google-drive-ruby