Refactor user controller, create action, mostly.

The gist of the commit are a few improvements in the
create action, where:

* long boolean statemenst have been wrapped in smaller more readable
  methods.
* the 3rd party user info creation has been extracted (still in controller)
* a small helper method for creating a new user from params (to reduce
  visual clutter)
* specs have been added where I came across untested methods/branches

Other changes are more trivial like formatting and whitespace fixes.
Hope this helps. Regards.
Merge pull request #705 from wpp/refactor_users_controller

Refactor user controller, create action, mostly.