Feature/notifications

Here it is - this took an entire week to implement, but I have the following notifications working:

* Question created
* Question answered

More notification rules will be easy enough to add - mostly, it's just a case of extending the `Question` or `Answer` model to return true or false to match a notification rule, and adding an email template.

Most of the complex functionality for this feature has been refactored into a gem called [disclosure](https://rubygems.org/gems/disclosure). The changes in this pull request cover the following things:

1. Adding notifiable actions for the `Question` model
2. Add mailer views for question actions
3. Extend models where necessary to react to Question actions
4. Add to User model to create default notification rules when an account is created.

@NzCub3y could you please take a look at this one, and ask any questions where I haven't been clear enough. Travis should pick up whether the build is still passing or not.