Issue 40: Iterate through VorbisComments

[AaronLenoir]

Adds the possibility to iterate through VorbisComments (without knowing the tag name).

// Iterate through all VorbisComment tags
using (FlacFile file = new FlacFile(@"Data\testfile1.flac"))
{
    var vorbisComment = file.VorbisComment;
    if (vorbisComment != null)
    {
        foreach (var tag in vorbisComment)
        {
            Console.WriteLine("{0}: {1}", tag.Key, tag.Value);
        }
    }
}