This allows for multiple notifications to be displayed on Android.  Simply add data with key "notId" and an integer value when sending via GCM.  Differentiate between different notifications by setting different notIds.  Defaults to 0 if not key supplied.

We should always send a notification this is better than never sending one

We can rely on the the other activity functions setting foreground to be true

Allow the plugin to know when the app is no longer in foreground so that it may receive push notifications

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).

* Don't check if we are in the foreground - that happens anyway in the
sendExtras() method (https://github.com/phonegap-build/PushPlugin/blob/master/src/android/com/plugin/gcm/PushPlugin.java#L111)
* Only display a message if the message has text to be displayed

Fixes #52 and undoes #3

* Put all the plugin code in the plugin namespace: com.plugin.gcm;
* Renamed to namespace GCM to gcm, namespaces should be lower case
according to the java style guidelines;
* Deleted the google gcm code and added gcm.jar instead;
* Added CordovaGCMBroadcastReceiver.java to enable GCMIntentService to
be namespaced in the plugin namespace instead of the client project
(which is is the default with google's GCMIntentService);
* Moved all java <--> javascript interop to PushPlugin;
* Moved all notification code to GCMIntentService (maybe this fits
better in its own class);
* Changed the way the plugin detects cold starts in order for it not to
crash when the back button is used to navigate out of the app;
* Changed the caching on the filesystem to a memory cache on a static
variable, which increases performance and simplifies code;
* Cut up some code into separate methods ;
* Added code comments;
* Updated the readme file.