Add search on keypress, clear input when value unchanged

[mbeard]

Added support for search on keypress and fixed issue with clearing out unchanged search value

removed clearing search on consecutive search (double enter) and unit test

replaced double enter with esc key listner

fixes #1244

[swilliamset]

Nice to have:

Also add comments above each number or create a constant so code is easier to read

var ENTER_KEY_CODE = 13;
var TAB_KEY_CODE = 9;
var ESC_KEY_CODE = 27;

instead of the Unchanged part of inputEmptyOrUnchanged add a escape event listener for e.which === 27
https://github.com/ExactTarget/fuelux/pull/1598/files?diff=split#diff-d47536a005af642c7d7777bfa3177960L92

else if (e.which === ESC_KEY_CODE) {
    this.clear();
}