[aerogear-dev] [aerogear-android] PR 94 API refactor

[Summers Pittman]

The main thrust of this PR is to make the Push APIs follow the 
factory/config pattern of Pipe, Authentication, and Store.

Here is a before and after gist : 
https://gist.github.com/secondsun/b01c3446095fb29a85e3
Here is a link to the PR: 
https://github.com/aerogear/aerogear-android/pull/94

I've also included a small bit of API cleanup.  For instance the 
PushRegistrar now keeps a reference to the PushConfig so you don't have 
to pass it into register and unregister.  Also, I extracted an interface 
from the Registrar object so in the future we can add support for non 
GCM push services.  (There are also two JIRAs to support that eventual 
goal).