Added ability to Remotely Dismiss Notifications

Added ability to remotely dismiss/clear notifications from the Notification window.

To do this, you can just set the variable "cancelNotification" in your GCM request (it can be set to anything) and if the notification is present it will disappear from the notification window. 



This uses the default Notification ID set as public static final int NOTIFICATION_ID.

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Relevant only if you used theNewMr code for multiple notifications here - https://github.com/thenewmr/PushPlugin/commit/728e975442e45668b7d809aec028396a9b023dfa

In order to dismiss specific notification Id's, just make sure that notId is set in your GCM request to whatever notification Id that you want to dismiss.

NOTE: I would make sure that you read the comments above about re-adding the line where the default NOTIFICATION_ID is set and changing the default notId value from 0 to the variable NOTIFICATION_ID. This is necessary in instances where notId is not set.

(So far this has only been tested on Android)