Add an embeded process manager

This adds the `/process-manager` API. HTTP requests beginning with this URL will be proxied to the `strong-pm` instances running as a sub process.

**Note:** the first request to this URL will start the sub process... all other requests will just be proxied to the running process.

/to @anthonyettinger @seanbrookes 
/cc @kraman
You didn't tell spawn to direct child stdio to the parent, so I don't think this will work.

I'd suggest using fork, and receiving the listen PORT via IPC, more robust.