Remove use of find_by_uid method on User.

Not all ORM's provide this method (e.g. Mongoid 3).  Replace its use
with a simple `where(:uid => 'foo').first` that's more widely available.
Update README to remove requirement of find_by_uid method
Merge pull request #19 from alphagov/remove_find_by_uid_requirement

Remove use of find_by_uid method on User model.