[#644] External User

* Initial scope documentation
Make engine generators work correctly.

`rails generate/destroy` now correctly place files in the engine rather than test/dummy.

* Move dependencies loading to gem file (lib/browsercms) which avoids loading all of them during rails tasks.
* Use Minitest when generating models.
[#644] External User API

Add a new API for handling users authenticated against external data sources.

* Introduce Cms::ExternalUser
* Extract behavior to PersistentUser class.
* Create a TestPassword strategy which demonstrates how to do external authentication. When enabled, any account w/ test/test can log in if enabled.
* ExternalUser's are assigned to 'Guest' group by default.
* Add ExternalUser#authorize to mark the specific groups.
* Test Password puts the user in cmsadmin groups.