Use getSystemService() to get the activity scope

Building on top of #70, which brought the great idea of using system services.

* Overriding `setContentView()` and manually inflating was not a good idea. It broke ActionBarSherlock support. It also created a useless extra context wrapper, for the sole purpose of providing the activity scope.
* Instead we let the activity provide the scope as a system service. Same goes for the application. More decoupling ensues.
* I also cleaned a few places in the doc that were leftovers from the `MortarContext` times.
Merge pull request #72 from square/py/better_activities

Use getSystemService() to get the activity scope