remove requirement for provideThis()

[codefromthecrypt]

per square/dagger#176 we can unwind some complexity that results in implementors having to add provideThis() to their Provider subclasses in the following fashion:

One thing you could experiment with is ObjectGraph.plus(). Perhaps you could build your application in two phases: build a small inner object graph with everything you need during bootstrap, then build upon that to create a more complete object graph.