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
@kbrock My recommended nginx config is 

```
accept_mutex_delay 100ms;
keepalive_requests 500000;
```

This config is often useful for benchmarks.