Revert "Remove username generation; use e-mail for login"

This reverts commit d816d7c5a1361870fe9168a1ebe2446b40c65515.

Conflicts:
	lib/outpost/model/authentication.rb
	spec/lib/model/authentication_spec.rb
Fix
Use Outpost.user_class
Spec updates
Return guard for generate_username; no need to validate e-mail presence