Add django-debug-toolbar

As it can do SQL explain and many other magic things while browsing the
API.

Related to https://www.pivotaltracker.com/story/show/76514752 [#76514752]
Turn on django-debug-toolbar on EMERGENCY_DEBUG

So we can use it on Heroku staging.
Merge pull request #63 from sealevelresearch/add-django-debug-toolbar

Add django-debug-toolbar