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

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).