Tilemap#paste diffX and diffY equations changed

This should correct an issue with copying and pasting sections of tilemaps to different x/y coordinates. Relates to this forum post: http://www.html5gamedevs.com/topic/3367-tilemap-copy-paste-issues/

I've added a QUnit test for this change. As it is dealing with data manipulation I found it useful. 

The tests highlighted another possible issue with tilemap copy and paste. Changing data in copied data will actually change the tilemap it is copied from. This is because the copy creates an array with the same tile references as the actual tilemap. Not sure of the best solution there as memory could be an issue.

I'd be interested in contributing further to Phaser and I was wondering if adding some more unit tests might be useful, what are your thoughts?