Add some improvements

* improvement: add several new tags for each album and track
* improvement: add several new tags (e.g. artist_sort, url) to the tracks
* improvement: add possibility to use '/' in dir-property, to allow subdirectory
               creation (e.g. %ARTIST%/%ALBUM%)
* feature: add possibility to name the first image folder.jpg, so that clients 
           recognize this picture, even though it is not embedded (Issue #12)
* feature: add multi disc support (Issue #14), this does right now just covers
           the handling of tags (discnumber, discstotal) and does not take 
           care about the filehandling

currently working more on the multi disc handling, probably you would like to wait. but i do like to have some input on the already done stuff. is this going into the right direction? what do you think?