Update README and add third-party license info.
Merge remote-tracking branch 'upstream/master'

* upstream/master:
  Add push handling for notifications from origin helpshift
  Implement basic Helpshift support in GcmClient
  Add GCM Client sample app
  Add google-play-services_lib
Add readme for GcmClient project, move version specific notes to version list.
Link projects, format version notes.
Merge pull request #1 from ideamonk/master

Update READMEs for Android SDK
Merge pull request #2 from helpshift/master

Readme updates