Add musepack decoder.

Add musepack decoder.

Original commit message from CVS:
* configure.ac:
* ext/Makefile.am:
* ext/musepack/Makefile.am:
* ext/musepack/gstmusepackdec.cpp:
* ext/musepack/gstmusepackdec.h:
* ext/musepack/gstmusepackreader.cpp:
* ext/musepack/gstmusepackreader.h:
Add musepack decoder.
* ext/faad/gstfaad.c: (gst_faad_base_init):
Make pad templates static.
* gst/typefind/gsttypefindfunctions.c: (mp3_type_find),
(plugin_init):
Add musepack typefinder, make mp3 typefinding work halfway stream,
which doesn't actually work yet because id3demux doesn't implement
_get_length().