Added No Color / Clear color selection capabilities

Sorry, had to reset my fork because the carriage returns were all messed up!  This commit fixes the issue.  

Details:
This allows users to clear a color selection, and introduces 2 new spectrum options:
allowEmpty: (defaults to false)
clearText: (text displayed to clear the color selection)

This is just an initial implementation. Everything to works, but I think it could be a bit cleaner. Open for suggestions!