remove _del call from _add

So that users can bind keydown, keypress, and keyup for the same combo.
fix purgeHotkeys to delete all

purgeHotkeys would previously miss the tail end of the array, because it was using a forward loop (angular.forEach), and the index was decrementing with each delete.
Merge pull request #27 from willhoag/multiple_binding

Multiple bindings for same key