Prepare Derek for use as an SDK/package

## Expected Behaviour

We should prepare Derek for use as an SDK/package so that he can be run on any platform - a Go microservice, AWS Lambda, Heroku etc.

This is the first step that makes Derek a package that someone can vendor in Go and then make use of in their own stand-alone way. #13

## Current Behaviour

Derek may "os.Exit" at several points in the code.

Derek's configuration may depend on certain characteristics of OpenFaaS.

## Possible Solution

* Try to vendor Derek as he is right now, see what happens.
* Compile a list of suggestions on this issue and ask for input
* Move all code out of `main` and into packages

## Acceptance criteria

* Test Derek spun up in a single Go HTTP server outside of a container
* Make sure there is no regression