Automatic setup with explicit fallback

I'm very excited about this!

In the general case, it makes installing the debug toolbar as simple as adding `debug_toolbar` to `INSTALLED_APPS`. (It isn't necessary to edit MIDDLEWARE_CLASSES any more).

However, users retain full control of the installation: they can perform the installation steps explicitly instead. This will be especially helpful for people with complex URLconf setups that hit limits of the toolbar's support for per-request URLconfs. It should fix issues #441, #421, #300, #277, #254, #84 and make it possible to fix #448.

@robhudson @dcramer @jezdez do you think this will work?