Added Delegate Callback Method For Tap Detection.

Similar to the [feature I added to SFNotify](https://github.com/sferrini/SFNotify/pull/1) that allows users to provide their own code to be executed when the notification view is tapped. 

However, this time, instead of using blocks (or closures since its swift), I decided to follow your existing protocol-delegate pattern and added a delegate method that the user can implement and is fired when the Notification View is tapped.

A gif is shown below:
![sfswiftnotificationtap](https://cloud.githubusercontent.com/assets/7833300/4800903/223171a8-5e2b-11e4-837f-653a7205c5c9.gif)