Replace attribute selector note with class selector, class demo & class test

Replaced attribute selector [name=name] inline documentation (as referenced doesn't work in HenrikJoreteg/Happy.js#42 and HenrikJoreteg/Happy.js#52) with class selector documentation and added working example of class selector to demo.html and class input selector to test.js
Did you mean to comment this part out? It breaks the demo on the second form.