First stab at adding push notifications for Android

High Level Changes:

* Added required service and receiver declarations, along with required permission sets to ```AndroidManifest.xml``` in the ```SalesforceSDK``` project.
* Added ```app_package``` to ```strings.xml``` for the sample apps, so that we can keep the common manifest declarations in the SDK's manifest file.
* Added permission sets to the sample apps.
* And, finally, added 3 new classes with the implementation details of push notification registration and message receiving infrastructure.

What's Left to Implement:

* The actual calls to ```PushService``` upon authentication (for registration), and upon logout (for unregistration).
* Fetching connected app perms to check if push notifications are enabled, before attempting to register.
* Testing of this whole thing against a legit SFDC connected app endpoint.

The recommended GCM implementation paradigm for Android is outlined [here](http://developer.android.com/google/gcm/client.html), if you'd like to read about the nitty gritty details.