Support for audiobooks

[IFelix94]

I've just started using Subsonic, great for music, but I'd like better support for audiobooks.

When adding a library I would like to be able to choose between music and audiobook, and in the case of a library of audiobooks:

* Specific metadata for audiobooks (narrator, description of the book, etc. see Booksonic)
* Support to the chapters of the .m4b files
* Automatic bookmark, and the possibility of automatically resuming from that point independently of the client