Handle different capitalizations of metadata in flac files

FlacInfo returns the tags as a hash. This means you have to know the exact key to use to extract the information you want - e.g. Artist, artist or ARTIST.

I've introduced a really small wrapper class to provide case-insensitive access to this hash and removed the 'or' logic which is no longer required.

I've also added tests which demonstrate this behavior. Note this required checking in a (small - 100K) flac file to the `test/support` directory so that I could be really sure that things worked.