Allow the EventMachine proxy port to be configured.

The port will default to a value of 0, which causes EventMachine to dynamically select a port per https://github.com/eventmachine/eventmachine/wiki/FAQ#wiki-can-i-start-a-server-on-a-random-available-port .
Explain proxy_port default magic number.
Merge pull request #11 from centro/assign-em-port

Assign EventMachine port
Merge pull request #40 from centro/centro_production

Allow configurable Billy proxy port