Import dkjson, lua-bencode, and lua-sha2

A couple of questions:

- Does the lua-sha2 Makefile look okay? We haven't observed any concrete problems, it's just that I'm not sure I haven't done anything stupid ;)
- these three Makefiles have so far been licensed under GPLv3, are v3 and v2 compatible, or should I make sure all past contributors are okay with v2?
- should dkjson be prefixed with lua-?
- should PKG_RELEASE be bumped for each package? The versions in this PR are the latest from seattlemeshnet/meshbox.git
- can I specify multiple maintainers?
I changed the md5sum, then ran `rm dl/dkjson-* && make package/dkjson/{clean,compile} V=s`, which succeeded.