Images and more link types supported

This includes #28, #30, #31, #32, #33 and #35. If you want to review them separately I think that their commits should disappear from below when you merge them.

The only thing that is relevant to this pull request is faa0197, which depends on the other hence the inclusion.

I've added support for a new type of `SpObject`, `Image`. All link functions `Image.getFromUrl` and `img.getUrl` is implemented. You can also get an image object form an `Album`, `Artist` or `Playlist`.

Some tab and spaces misses are also fixed.