Add method to browse by string id

This is the result of the following discussion:

https://github.com/SoCo/SoCo/pull/192#issuecomment-53198218

Hopefully everyone is happy with this and it can be flagged for 0.9?

Thanks, Rob
I'm not sure I agree with that, the names used are local to the Soco class (rather than global), they are specific names for the different operations as I understand it.  Previously they were scoped to the method get_music_library_information as that is the only place they were used, now they are increased to the scope of the class as it is the class that is the only place they are used.

Hope that makes sense!

Rob