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