Add support for parsing the disc number tag from metadata and use of it in the database. Patch originally from FS#4961 with some minor tweaks by me.

git-svn-id: svn://svn.rockbox.org/rockbox/trunk@14154 a1c6a512-1295-4272-9138-f99709370657
diff --git a/docs/CREDITS b/docs/CREDITS
index ff9658e..e84521b 100644
--- a/docs/CREDITS
+++ b/docs/CREDITS
@@ -309,6 +309,7 @@
 Rene Allkivi
 Tobias Schladt
 John Zhou
+Charles Voelger
 The libmad team
 The wavpack team
 The ffmpeg team