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
#### my benchmark environment
- cpu core * 4
- simple hello world benchmark

#### nginx worker_processes config
- worker_processes  auto(4)
  - result: 91417.99req/sec
- worker_processes 8(max_thread *2)
  - result: 73174.72
- worker_processes 12(max_thread *3)
  - result: 68691.84