Isolate executor capabilities of the LocalStore client.
Isolate callback handling in a utilities class.

Since the way callbacks are invoked is similar through out the entire
SDK, this functionality should be collected in one helper class and
re-used wherever possible.
Let VolleyRequest use the CallbackManager.

The VolleyRequest has a slightly wider callback spectra, hence
there is a need for a VolleyCallbackHandler, which is hereby
implemented and used.
Let LocalStoreRequest use the CallbackManager.
Define the Push API.
Implement the Push interface by the FayeClient.

This implementation enables push capabilities through the Faye
infrastructure.
Implement an HTTP long-polling transport layer.

This implementation relies on the Volley HTTP library.
Create push event domain models.
Prevent local build scripts and artifacts from being added to repo.
Merged with master.