Added getString method to setContentTitle and setTicker

What's changed:
ContentTitle and Ticker can now be set from the GCM server via the "title" field in the "data" parameter (appName is no longer hard coded).

I am not really sure why appName was hard coded into these parameters, as most examples for this plug in allows you to set a title. For example, from this tutorial:
http://devgirl.org/2013/07/17/tutorial-implement-push-notifications-in-your-phonegap-application/

The node.js server example is as follows:
**message.addData('title','Push Notification Sample' );**

But the title will not be set and displayed on the device as appName is the hard coded value in _GCMIntentService_