Add step types: moveMouseByOffset leftClick rightClick

* add moveMouseByOffset step type
* add leftClick step type
* add rightClick step type
* 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