Recording - automatically add sites to containers as you browse

[mckenfra]

Currently it's impossible to add an assignment if the URL immediately redirects. E.g. if you enter google.com in the address bar, it will immediately redirect to www.google.com. If you then open the MAC popup and click on Always open in ..., the assignment for www.google.com will be created, but not for google.com.

This pull request adds the ability to record assignments automatically as you browse. So in the example above, it will automatically add both google.com and www.google.com. See screencast below.

This pull request also improves the MAC messages that popup over your browser window, e.g. when you click Always open in .... Previously the messages sometimes were mis-positioned or did not appear. Now the messages appear correctly, and multiple simultaneous messages stack with a smooth animation. See screencast below.

Unit tests included.

[jonathanKingston]

@TanviHacks I really think this is a pretty cool feature. How are we handling new features like this now? Do they have to go through design?
I'm happy to code review if we want to merge this.
Perhaps the better solution is provide the correct APIs so another extension can do this instead and MAC becomes and extension point itself?

[dannycolin]

Closing in favor of implementing an API to manage website assignments in Firefox itself.