Track Go library dependencies so that builds are repeatable

Add dependency management with gpm, and more thorough Makefile.  Based on https://github.com/blalor/go-template.  

I understand dependency management with Go is a controversial topic, but I'm more interested in making stuff work than with dogma.  I think it's important to have some kind of audit trail so that you can find the point where a changed dependency breaks your application.  Having this change in makes me more comfortable using Registrator in my environment.

I attempted to preserve the `release` target, as well as the Docker image build.  Those should still work fine, but only if you're building on Linux, which I am not. :-)