Migrate from legacy gcm to Firebase

[miguelreiswildlife]

Main changes done:

• Create a PushClient interface that makes more sense than the old GCMClient interface
  • Implemention of the interface uses firebase SDK
• Created a new implementation for MessageHandler
  • Created a new one instead adding a new client to the current one because firebase works very diferently in comparison to GCM.
• When the MessageHandlers map is instatiated, it will use the new if there are firebase credentials for the game in question
  • Otherwise it will let the GCMMessageHandler instantiate the client as was done before
• Firebase does not return the response of pushes async, but I tried to generate the same feedback and reports as done in the old handler