backend class hierarchy: first draft
default backend class
using backend instead of the orm
pep8
adjusted import notation
decoupled message storage from message backend
added archive_list method to MessageBackend
fixed tests, now using backend instance
base redis attempt

Conflicts:
	stored_messages/api/default.py
	stored_messages/storage/default.py
moved api module out of a package
moved out storage module from a package
added redis backend scaffold
added redis to optional requirements
removed unused module
added Redis settings
more on redis backend
renamed default backend
uniformed interfaces
added exceptions for backends
storage class revised