Add support for jQuery selectors (uses window's jQuery)

This allows "jquery" and "sizzle" locator prefixes, which delegate to the window's jQuery.  Probably not suitable for merging as-is; I think it would be better if jQuery was provided by selenium2library.

But it is easy and probably* works, so I thought I'd share.  Maybe it will inspire someone to enhance it.

* I have the change working on a work computer, but due to in-house restrictions, I had to type it in by hand on another computer in order to push to github.  The other computer doesn't have robot, so I haven't actually tested the re-typed-in code.... sorry!