added ability to set images to buttons

Ignore-this: 5037ac18ff33beda341d5a3988277e8c

darcs-hash:20140206194723-c21fe-bce22c52fadf4baad2afb11c65bcd533493286da
fixed bug where if you passed nil it would crash

Ignore-this: e4c1c80318857dbc92b6831cdf19536e

darcs-hash:20140206194803-c21fe-4b6f33be641aa057fd9b4accecc75e9f1d844779