Added go.mod file to migrate to go modules

[vinayarv]

Currently I'm unable to import this in my project as this doesn't have go.mod file. This MR addresses it by adding go.mod file. Here are the steps that I followed:

1. Forked and cloned the repo.
2. ran go mod init github.com/openvenues/gopostal
3. go build ./... and go test ./... just to make sure all works.
4. Published the new release by adding a tag.
   git tag v1.0.0
   git push origin v1.0.0

[vinayarv]

I think the release must be published on this branch after this merge request is merged.

[connormckelvey]

Can this get merged in? I'm using go modules as well and cannot import this library