Module changed to class ( now multiple applications are supported )

I've modified module structure to include an Application inside GCM and APNS module. This way easily you can create an application object with push configuration and call send_notification/s methods. 
I've updated README.md too in order to explain the changes and how the library should be used from now.

Useful to solve this issue: https://github.com/NicosKaralis/pushmeup/issues/3