Add `submit` element instance method

The `submit` method was only available as a `webdriver` static method.

Considering that:
> The submit command may also be applied to any element that is a descendant of a FORM element.
(from the [JsonWireProtocol](http://code.google.com/p/selenium/wiki/JsonWireProtocol#/session/:sessionId/element/:id/submit)) 

It is quite nice to be able to fill a field and submit it on the same reference.

This changeset adds `element.submit`, mapping it to the corresponding `webdriver` method, just like other mapped methods.