Implement new GCM features presented at Google I/O'13

[ToothlessGear]

• upstream messaging (including user notifications)
• persistent connection
• Chrome Notifications

[hypesystem]

A pull request for user notifications has been submitted: #96.

[hypesystem]

This issue should be split into (at least) three separate issues.

[hypesystem]

I have created a separate issue for user notifications: #107

[hypesystem]

Upstream messaging and persistent connections are intrinsically connected: https://developer.android.com/google/gcm/ccs.html#upstream

Chrome Notifications do not differ from any other notifications, and are actually already supported (although sending upstream notifications from Chrome obviously isn't).

[hypesystem]

I have split out the CCS/Upstream messaging into a separate issue, #109. With that, I can close this issue (as the parts have now been split out).