Implement RSA encryption

[Mechazawa]

Recently the pubkey parameter for the /gcm route on the pushjet-api was added. This adds the ability to encrypt messages sent through GCM. This should prevent data mining and content analysis. The main issue right now is key generation. Large keys take a long time to generate. Possibly an option to change the size of this key would be needed (512, 1024 [default] and 2048). The other problem is that a lot of android devices/ROMs have major issues with random number generation. Some investigation into this will be required before proceeding with the implementation.

Currently I'm considering using the most straight-forward way of doing it and hoping that it will be secure enough

[Mechazawa]

Topic branch is crypto_gcm