Singleton and Member cache cleanup

All Singleton methods are no longer available:
- dispersy.has_instance
- dispersy.get_instance
- dispersy.del_instance

The Dispersy instance is available directly from the LaunchManyCore,
or though the session instance (session.lm.dispersy).

Also, DummyMember and Member instances should only be created through
Dispersy using:
- dispersy.get_temorary_member_from_id (returns a DummyMember)
- dispersy.get_new_member
- dispersy.get_member
- dispersy.get_members_from_id
- dispersy.get_member_from_database_id