Use 'class' and 'for' for DOM property names

React fixes inconsistencies in the DOM and avoids being held back by older browsers (to the extent possible). The only reason that the DOM authors chose `className` and `htmlFor` was because the JS engines of the time didn't support using `class` and `for` as unquoted property names.

Changing React to use `class` and `for` agrees with our principles of improving the DOM and using modern JS, and is what most people expect when first starting with React.