Fix name of argument to buttonDown/Up and document it in README

The button argument to `buttonDown` and `buttonUp` should be `button` instead of `number`:
* <a href="http://code.google.com/p/selenium/wiki/JsonWireProtocol#POST_/session/:sessionId/buttondown">/session/:sessionId/buttondown</a>
* <a href="http://code.google.com/p/selenium/wiki/JsonWireProtocol#POST_/session/:sessionId/buttonup">/session/:sessionId/buttonup</a>

I fixed this and also updated the relevant documentation in the README.