Add luajson into benchmark

[v1993]

I'd be interested to see https://github.com/harningt/luajson in performance comparison. Reason behind that is because I use it in my projects due to being packaged in Ubuntu as lua-json package.

It uses LPeg for its parsing (without non-LPeg fallback), so it would be great to see how well pure-lua lunajson holds to it.

[tst2005]

(typo in issue title: s/bechmark/benchmark/ )