Update to add Google Play badge

Added the official Google Play large badge with a link to the OpenShift Mobile app. Resized PhoneGap badge to height of Play badge.
![screenshot 2014-04-14 23 51 37](https://cloud.githubusercontent.com/assets/632932/2703728/b77db6fc-c459-11e3-91b3-fecead06c679.png)