Add step types: moveMouseByOffset leftClick rightClick

[asauber]

• add step type moveMouseByOffset
• add step type leftClick
• add step type rightClick
• fix calling convention of mouseOverElement
  • if a single argument is passed (an element reference), then the default behavior of selenium server is to move to the center of that element
  • passing [el, null, null] raises a NullPointerException in the server
• update the interpreter to allow Number arguments by selectively calling string.replace on tr.do arguments

[asauber]

@Zarkonnen This is failing the build because of Sauce creds. Would be interested to see if it passes otherwise.

[Zarkonnen]

Yeah, it'll most likely work fine. Could you also add these new step types to Builder? Otherwise, they're of limited usefulness.