Add simple Dock API.

Added a Dock command with two methods:

* `dock.badge` — does not work
* `dock.setBadge` — does work

I am hoping I've missed something pretty trivial on the badge getter. I'd rather it have been a property rather than a method but unsure how i.e. in the JavaScript:

    var badge = macgap.dock.badge;

not

    var badge = macgap.dock.badge();
    // nor
    var badge = macgap.dock.getBadge();

Overall this is surely a huge step forward — thanks for the amazing work! — but I admit I'm still getting my head around everything. I'm keen to see an example of callbacks, both from a viewpoint of receiving, and of sending-data-to. Maybe menus would be a good example? An NSTask implementation would be awesome, but perhaps on the complex side for an example.

One other thing that I'm unsure of right now is the use for the javascript for each command e.g. `dock.js`. Previously there was no JavaScript at all for each command, and I'm not sure what it's for now?