Add an option called renameAttrs

I found it useful to be able to rename certain attributes. For example, the following JSX:

    <div class="hello"></div>

would be converted to

    h('div', {className: 'hello'})

when transformed using `options={renameAttrs: {'class', 'className'}}`. This allows people using [virtual-dom](https://github.com/Matt-Esch/virtual-dom) to write in a style that matches HTML, instead of writing something like `<div className="hello">`.

Along with the change, I've also written the unit test and updated the inline-documentation and README for this new option.

Please let me know what you think!