Add #ab_assigned to adapters.

* Allows retrieval of assigned alternatives per participant.
* Cherry-picked from de9b36ea6ad3c4f21552b07fd73129913e01d157

Conflicts:
	lib/vanity/experiment/ab_test.rb
added ability to get an array of the experiments a specific identity is assigned to, as well as a controller action to display those assignments

Conflicts:
	test/playground_test.rb