Remove the Contact domain object.

This object has no real counterpart in the API. The API rather returns
user profiles or bylines (more on these in the next few commits).
Make the User domain object Parcelable.

The Parcelable interface is Androids serializing mechanizm. In the context
of the Android runtime it's much more compact and efficient compared to
any conventional Java serialization. As of this commit the User domain
object, along with all its childrent, are now conformant to this type of
serialization.
Make the Push domain object Parselable.
Add the Byline domain object.

This is also Parcelable.
Make Presence domain object Parcelable.
Make Conversation domain object Parcelable.
Make ConversationProvider API v2 compatible.

The second version of the API will return the conversation domain
object when one creates a new one, not a custom result object as
did the old implementation of this provider assume.
Make Profile a separate, top level class.