Removeable round 2

This pull request replaces #536. has been rebase and test.html update with POC. 

This adds the ability to set a data-removable="true" flag which allows the user to clear their selection.

When a selection is made the caret or arrowIcon is replaced by a removeIcon, when clicked the selection is cleared and the caret returns.
And it's easier to test and debug with the unminified bootstrap-select.