Update to new GCM library on Android

[ChristianeMussmacher]

requires Maven Plugin update

[j-era]

Migration to Gradle Build System

Todos:

• [x] Release Build!
• [x] Test Build + Hockey App!
• [x] Proguard
• [x] git-commit-id-plugin
• [x] Maven build compatibility
• [x] Check for runtime failures
• [x] Wiki + Docs
• [x] Fix avatar badge layout
• [x] Use flavored hoccer icon for search
• [x] Whitelabel + Cleanup
• [x] Fix docu links

[elRadish]

Reviewed with @j3nsm4n , merged in 9cfbccd