Add CentralAuthenticationStore class for Spark modules.

Spark modules are meant to not have the code for working with one API provider in one class. As such you might split handling of user profiles from handling of posting messages.
This structure might require you, depending on the API provider, to supply authentication data to both objects if you use both within one request. In order to make this less troublesome
we introduce a CAS class storing this authentication data. The Spark modules use the CAS to request and store authentication data and as such they are immediately shared between all objects that might need them.

Unit tests, sample implementation for facebook.