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 ;)
It IS a good place for a ternary operator.
Keep in mind though, that switching it to a ternary operator is more for cleaner-looking code and less about optimization. Compilers do a lot of tricky stuff, and they're very good at optimizing small code blocks like this. Probably no real difference.