Display current live sessions

PT story: https://www.pivotaltracker.com/story/show/74032372
Demo: http://yaro.herokuapp.com/hangouts

Major changes:
- updated the `Hangout` model with: project, event, host, participants, youtube_video_id
- `Project, Event, User` - has_many hangouts association
- introduced `NullUser` class to handle anonymous users
- introduced `HangoutPresenter`
- `UserPresenter`: added `user_avatar_with_popover` method
- added route for `/hangouts/index`, `Hangouts` index page

Minor changes:
- added config to guard to focus on @wip tagged scenarios
- `hangouts_helper`: use project_id instead of project
- default scope for `Hangout` is created_at DESC
- updated `Events` and `Projects` show pages and specs to account for Hangout model changes
- updated `_hangout_button` partial to account for Hangout model changes
- added `_user_avatar` partial and shared_example spec to render user avatars
- added steps definition for `hangouts_path`
- unified the use of `Event.title`, `Project.name`, `Hangout.topic` to use `title` in views 
- avatars show extra user info on hover and redirect to user_profile on click

Refactorings of existing code:
- `hangouts_controller`: identify local requests by ip rather than hostname
- 'BasePresenter': added delegation for `UrlHelper`, added method `presents'
- `UserPresenter`: use `presents` method instead of 'alias_method'
- adjusted step definitions for avatars
- changed to using User and hangout factories in specs and steps

Full version:
![image](https://cloud.githubusercontent.com/assets/5670940/3874617/29e8755a-2144-11e4-9cde-54d12d72b2e4.png)

Responsive version:
![image](https://cloud.githubusercontent.com/assets/5670940/3874720/b82b14d4-2145-11e4-8826-63743ea51c01.png)


NullUser delegates anything it does not implement to User anyway.  I did not want to subclass from User model to keep free off the overhead.  Also, subclassing from User model will allow to persist it.  We would not want that, would we?  Or we might?  not sure here.