Consider using go modules

[SEJeff]

Upstream go has created a system called modules for dependency tracking so users don't need to vendor dependencies. Consider using this instead of Gopkg.lock and vendoring.

[rochaporto]

I've changed to use go modules in most of my go libraries, will do this one as well. Thanks.

[rochaporto]

The PR has a bit more than just the migration to go modules - a new Makefile and an updated layout for the git repo.

[SEJeff]

Great stuff, thanks so much!