Split out bencode dissector and use it in the bzr dissector.

Change-Id: I5b2ef173faf7670fad64298d67f2aaf055b8d6d8
Signed-Off-By: Jelmer Vernooij <jelmer@samba.org>
Reviewed-on: https://code.wireshark.org/review/82
Reviewed-by: Evan Huus <eapache@gmail.com>
Tested-by: Evan Huus <eapache@gmail.com>