Universal navigation drawer

This is an implementation of a universal navigation drawer. It provides an activity, ```NavigationDrawerActivity```, that can be subclassed to provide a navigation bar in that activity. I strived to keep as much of the logic of handling the navigation drawer as possible in the super-activity, while still allowing methods to be overridden for manual handling of events.

If anyone has any suggestions on how to improve the framework that's set up, let me know. I still consider this in early development, though it's largely complete and functions as it should as far as I can tell.

Addresses #44.