Fix #15.

Closes #17

LGTM given by: @nabrozidhs

Squashed commit of the following:

commit acbbca9f8a24ed7a09bf934c0a31381fbfe1816b
Author: Alex <nabrozidhs@gmail.com>
Date:   Sat Aug 23 17:48:06 2014 +0100

    Fixed typo for OnAdClosed callback.

commit b6eeeb5011adf581633af9c8e371fe8c987f4b65
Author: Alex <nabrozidhs@gmail.com>
Date:   Sat Aug 23 17:36:44 2014 +0100

    Fix #15.

commit 1b66a1832a760353be402704e102269d98925899
Author: Alex <nabrozidhs@gmail.com>
Date:   Sat Aug 23 17:34:18 2014 +0100

    Handling all AdListener callbacks from Unity.

commit 7120abd02aed319c511ed1e52f8f75476811513f
Author: Alex <nabrozidhs@gmail.com>
Date:   Sat Aug 23 16:01:31 2014 +0100

    Handling all AdListener callbacks from the java side.