artist: getAlbums and getAvailableAlbums

[LinusU]

This includes support for getAlbum() and getAvailableAlbums(). I went thru great lengths to get getAvailableAlbums() to return the same list that the spotify desktop client shows, but there is one step left which is documented in the source.

This also includes #32 since it depends on that one.

[Floby]

I'm unclear on what getAvailableAlbums should return, I don't remember seeing it in the libspotify docs ?

[LinusU]

Sorry, I should have been more clear. I took the liberty of adding in the getAvailableAlbums() to make this library more useful out-of-the-box. Calling getAlbums() will most likely give you a very long list with many duplicated albums, used in different regions. I took the time to try and figure out how the Spotify client does when deciding which albums to show to the user when it navigates to an artist and copied the functionality.

I thought that it might be useful to other aswell but I'll take it out if you don't want it.

[LinusU]

(also, it depends on #32, not #7... I was very tired when I submitted this)