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?
See https://github.com/openwrt/packages/pull/280  I think you had leftovers in your build dir, which is why you didn't notice that wrong paths, and the md5sums not working.