add ruby/ngx_mruby framework

Hello,

I was able to get this installed and running in a vagrant dev env.
It does exit after running the test, so there may be a bug in my code or the current suite.

Any pointers would be appreciated.

thanks
@methane it doesn't. ``workers=threads*2~3 `` is better for ``db.execute()``, but that's may be overhead for text plain or json benchmarks. The priority of text plain benchmark is high for ngx_mruby. But your opinion is right, thanks.