Added Touch Callback Feature

Users can now provide some code to be executed when the notification view is tapped. I added a method that creates a Notify View which takes in a block parameter to be executed when the notification is tapped.

I used this control in one of my projects and needed to do something when the user tapped a notification. Since it wasn't already implemented, I added it.

Below is a gif showing it in action (sorry for the low quality):
![notificationobjc](https://cloud.githubusercontent.com/assets/7833300/4800686/bdffc006-5e28-11e4-92a0-97f6ee777b46.gif)