Add git submodule for lua-cjson.

Reference commit  4bc5e917c8cd5fc2f6b217512ef530007529322f
(2.1.0 tag, stable).
Add lua-cjson support in box.cjson

Code review of cjson branch by Dmitry E. Oboukhov.
Hand-picking the changes since the original branch
is based on import of lua-cjson into Tarantool
revision tree, and this branch is based on a git
submodule.

This patch implements
https://blueprints.launchpad.net/tarantool/+spec/lua-cjson