Extract build settings to central file.

Refactored build specific settings into one Build.cs file in Phoebe
project. This allows us to easily override various settings in one place
and not expose private settings to the world.
Add IPushClient to Pheobe.

IPushClient and PushRestClient is a small abstraction over HTTP APIs to
register/unregisters the users device for push notifications.
Register application context with ServiceContainer.

Add reference to the application to ServiceContainer so any piece of
code could get a reference to the application context.
Add int? helper functions to SettingsStore.
Add WakefulBroadcastReceived.

Ported WakefulBroadcastReceiver from support library to C# as we really
don't need the overhead of referencing the support library.
Add Google Play Services component.

Added reference to Google Play Services component and required manifest
configurations to use Google Cloud Messaging.
Add GCM push message support.