RPM packaging and init script

I know @progrium is mostly targeting registrator toward containerized deployment, but I think in some cases it makes sense to run it as a process on the host running Docker and Consul: running registrator in a container requires configuring Consul to listen on all interfaces which I would like to avoid if possible.

This follows the standard RedHat service template for pre-RHEL7 hosts (ie SysV init script instead of systemd).

This is built on the branch for #48, because it uses that Makefile as a baseline.