User profile youtube

Pivotal stories
https://www.pivotaltracker.com/story/show/65429770
https://www.pivotaltracker.com/story/show/64993078

Youtube integration:
- `authentication_controller`: methods to handle authorizing youtube API, linking and unlinking youtube channel
- `omniauth.rb`:
    - added keys for local testing with github and gplus
    - added scopes for Youtube API
- `users_controller`: fetching user videos
- `users_helper`: 
    - helpers to handle video links rendering and channel linking/unlinking buttons
    - Youtube module: API calls
- migration to add youtube_id column for User
- json fixtures for youtube responses

Profile page:
- User show view + specs
- cucumber scenarios + steps
- minor additions to user_steps and basic_steps to support youtube testing
- `users.js`: functions to support selecting videos on user profile
- `UserControlle` + specs

Non story related changes:
Gemfile: 
- added `better_errros` and `binding_of_caller` for rails pretty error output and online debugging
- updated `zeus` version to support RubyMine
- commented out debugger, as it breaks RubyMine debugging

`users_helper` -> `application_helper`: moved date_format helper
`env.rb`: disabled external web cnnections for webmock