Added destroy handler to remove the select2 when the parent element is removed from DOM

If I have <select ui-if='some-condition' ui-select2 ... ></select>, when some-condition is false, the select2 generated artifact is left behind. I am not sure if there is a better way that doing this in a $destroy handler or I am doing my code the wrong way. - Thx
Merge pull request #20 from sudhirrs/patch-1

Added destroy handler to remove the select2 when the parent element is removed from DOM