Add support for multiple genres in tags

[pollyzoid]

This converts the comma-delimited string into a list when writing tags
and vice-versa when reading.
This way it adds support for media browsers separating genres without
breaking database storage or querying.

Should help with #119

[pollyzoid]

Multiple genres doesn't seem to work for MP4 or ASF audio files, so I left them intact for now.

[sampsyo]

Very cool! Thanks for taking the first stab at this; we should definitely move closer to real multi-genre support.

I'm not 100% certain about using commas to separate the genres for this case, though. It would be great to be able to losslessly distinguish between a string with commas in it and a list of strings. So, concretely, it would be great if mf.genre returned a list of strings.

This would, of course, be a headache for all the client code that currently expects it to be a string, though. One possible resolution would be to introduce a genres field that's list-valued and keep the genre field string-valued. The latter could either hold the first genre or use the comma-separated approach you have here.

Does that sound reasonable, or is it just crazy? (FWIW, there is precedent in MediaFile for providing two views onto the same data—see the date fields.)