Merge pull request #6 from sjhworkman/master

Allow values for TrackNumber > 0; map TrackNumber to external index (from https://bugzilla.mozilla.org/show_bug.cgi?id=804372)
Use ne_map_track_number_to_index in nestegg_get_cue_point (from https://bugzilla.mozilla.org/show_bug.cgi?id=832018)