support delete via click, multiple entries, sort for single + bit of cleanup

New Features:
* remove elements via click on the x #5 
* add multiple values of the same type #6 

Technical changes:
* removed options from the state. You can now get the availalbe options via _getAvailableOptions. This should also fix #9 
* selectedValue was removed and replaced with selectedIndex. This is necassary cause a value can now be in values multiple times. Also makes the code much cleaner
* _sort was moved to the mixin to allow it to be used by the single and the multiple choise
This should be an anchor link so that it is more obviously clickable.