Update existing tests and provide new ones to test new paradigm.

Refactor new set_many and update XBlock version number.

Added self to authors.

Conflicts:
	AUTHORS
	CHANGELOG.rst

stores_state is now only used in a single test, in conjunction with
has_score. In practice, it's never the case that stores_state is false
when has_score is true, so we can delete stores_state entirely, and just
use has_score for the grading test.

Use the XBlock library as the base for XModule, so that we can incrementally rely on more and more of the XBlock api