Add test functions and instructions to verify installation

[LucasRoesler]

What

• Add test publisher and receiver functions and instructions for how to
  use them to test the installations of the nats-connector. This
  should make the installation easier for users.

@alexellis the test functions will need to be built and pushed to the OpenFaaS Docker Hub

Resolves #4

[alexellis]

Do you need to commit the vendor folder when using modules?

[LucasRoesler]

@alexellis i left them there because i thought it would be easier than documenting the requirement to use -b GO111MODULE=on during faas-cli build. The stack file doesn't support build args yet you have to pass the build arg to use go modules in the template. That is why i created some new templates for my personal use, but i figured you would not want that included here.

[alexellis]

Thanks for simplifying the example 👍

Given that the OpenFaaS contributing guide asks for dep still, I was surprised to see go modules used anyway. I guess we can leave it as is.

The user will be running bespoke commands for testing this, so I don't think using --build-arg is a problem.

[alexellis]

I'm running faas-cli build/push for the images as requested.

[alexellis]

The images are up now 👍