Added support for button colors being specified as hex strings, rgb string, and nicknames, see https://github.com/phetsims/sun/issues/53.  This required a mod to Scenery code.  Also, the button demo has been modified to use these specs as a test.
Added support for button colors being specified as hex strings, rgb string, and nicknames, see https://github.com/phetsims/sun/issues/53.  This required a mod to Scenery code.  Also, the button demo has been modified to use these specs as a test.

phetsims/scenery,phetsims/scenery,phetsims/scenery