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 noticed that the methods on Device don't really pertain to commands, but are more for appium to manage the device.....? Thought here was more appropriate.

I do agree that instead of manually adding all the inherited functions, devices should just extend `common.js`