Add JavaScript InfoPlugins

Hey.

I'm submitting an almost ready implementation of javascript bindings for info plugins.

If you want your ```FoobarResolver``` resolver to provide info system functionality, you can simply add a ```FoobarInfoPlugin``` to the InfoSystem by calling ``Tomahawk.InfoSystem.addInfoPlugin(FoobarInfoPlugin);`` from the resolver's init method (or wherever you want really). Its implementation would look something like this:

``` javascript
var FoobarInfoPlugin = Tomahawk.extend(Tomahawk.InfoSystem.InfoPlugin, {
    supportedGetTypes: [
        Tomahawk.InfoSystem.InfoType.InfoArtistReleases
    ],
    getInfoArtistReleases: function(infoHash) {
        Tomahawk.log("currentInfoPlugin.getInfoArtistReleases");
        return {
            newMaxAge: 2419200000,
            criteria: {
                artist: infoHash.artist
            }
        };
    },
    requestInfoArtistReleases: function (criteria) {
        Tomahawk.log("currentInfoPlugin.requestInfoArtistReleases");
        return {
            maxAge: 0,
            data: {
                albums: ['NO REAL ALBUM', 'Y U NO ALBUM']
            }
        };
    },

    supportedPushTypes: [
        Tomahawk.InfoSystem.InfoType.InfoNowPlaying
    ],
    pushInfoNowPlaying: function(pushData) {
//         $.ajax({
//             url: 'https://someserver/scrobble',
//             data: {
//                 artist: pushData.input.artist,
//                 title: pushData.input.title
//             }
//         });
    }
});
```

You can either return data synchronously or a promise. Your return value is automatically wrapped in a promise if you return synchronously. No chance for zalgo!

There's certainly room for improvement: for example the InfoSystem bindings could/should only be added to the javascript frame on demand if specified in the metadata.json.

I haven't done any Tomahawk development in a while, be nitpicky about coding style and everything.

Best regards,
Domme
Btw, it's probably not only about our code but also about qtwebkit license ...