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
Use `this.props.valueField` since it isn't necessarily the attribute value