replacing deprecated source :rubygems and replacing with https://rubygems.org
Improving User queries

it is now possible to generate a PFUser.query object by calling User.query. A simple User.all method was also implemented.

It is now also possible to instantiate a User object by providing an existing PFUser object.

Readme updated to match the changes.
Merge pull request #22 from DiogoAndre/User

Improving the User model behavior