added custom events

Hi there!
This pull request will allow to change the state of images not only on mouseenter/mouseleave events but also on custom events.
This is made with special custom events "BWon", "BWoff" and "BWtoggle" (the best method I considered for saving existing project structure).

The request includes: change of jquery.BlackAndWhite.js files, minified version change, readme addition and special demo page for this case.