Getter functions for Tilesheet

Added getTileCenter, getTileRect and getTileUVs to reflect
openfl.display.Tilesheet.
This necessitates adding a UVs array but adding tiles is rare and
overhead is negligible.
Upside is Tilesheets are far less opaque. Downside is users can shoot
themselves in the foot if they want.
Merge pull request #27 from furusystems/master

Getter functions for Tilesheet