Hande 400 errors in pyrax.identity.create_user

Previously, 400 errors were being silently ignored, so
pyrax.identity.create_user("name", "invalidemail") would return None. In
reality, this was getting an HTTP 400 error with a message about an
invalid email address being passed in. We now check if the HTTP message
includes text about invalid email, then raise pyrax.exc.InvalidEmail if
so, otherwise raise a generic pyrax.exc.BadRequest.
Merge pull request #193 from briancurtin/working

Hande 400 errors in pyrax.identity.create_user