Android: Activating a notification from outside the app creates a new instance of the cordova activity

When a notification is activated but the plugin is not active, an intent is created to activate the main activity:

https://github.com/phonegap-build/PushPlugin/blob/master/src/android/com/plugin/gcm/PushHandlerActivity.java#L60-L62

Since the default `android:launchMode` [android docs](http://developer.android.com/guide/topics/manifest/activity-element.html#lmode) causes a new instance of the activity to be created, a new webview is created even if the app is already running in background.

The simple solution is to change the launchMode so that a single instance is used.

This pull request adds documentation to the README.md and the plugin.xml so the developers are aware of this, since cordova does not allow adding the attribute automatically on plugin install.