Add Settings api, and xpathCompression setting for Android

Same as last, time, you need to use my fork of wd.js to run the tests...
I don't see the point to the extra complexity if we're already having to maintain logic such as `if (update.key === "xpathCompression") {` This will apparently be the first time [EventEmitter](https://github.com/appium/appium/search?utf8=%E2%9C%93&q=require%28%27events%27%29.EventEmitter&type=Code) is used for appium code. I tend to prefer simple solutions and then we can evaluate something better once we have more than a single setting. Even if we had 50 settings though, I'm still not persuaded about the emitter. I don't have any experience with it though so maybe it's normal for node code.