riemann-nginx-status

This pull request is a new script to hit an Nginx stub status endpoint and report metrics back to Riemann.

Remaining concerns:

 - This needs configuration through nginx. There's a link in a comment, but should it be explained more fully?
 - The code probably sucks pretty bad, and needs to be reviewed by someone who knows what they're doing with Ruby. I know *enough* to be dangerous, but not much more. I'll make any changes needed.

All that said, I've pushed this out to our servers already and it's working merrily along, pinging and reporting.

(implements #31)