Settings

added the `settings` api, plus implemented the setting for xpath compression.
added docs and tests

will squash once approved of course,

Oh, also had to add to wd.js to get the tests working. If we are ok with my implementation I'll push there. In the meantime you can clone [my fork of wd.js](https://github.com/jonahss/wd).

And then I'm going to go and update the rest of the clients....

(of course I'll squash, once we're through here ;)
I like the defensive coding here. :sparkles: 

If settings doesn't exist, is that an input error or a programming error?  If it's the latter, this is a great example of where an assert is perfect: it's more compact than your ```if``` and will catch any regressions.