Add go.mod file (go module support)

[smyrman]

Experimental Go module support has landed in the Go 1.11 beta https://groups.google.com/forum/#!msg/golang-dev/a5PqQuBljF4/61QK4JdtBgAJ. This introduces

It would be nice to add a go.mod file with dependency constraints for rest-layer around the time when Go 1.11 is released, and of course to have it in place before the v0.2 release.

This should be a relatively easy task in itself, although there may be some minor issues here and there.

[smyrman]

Resolved by #210.