Refactor build-release.sh

I've refactored the build-release.sh script so that there are bash functions representing each of the phases of building the release tar.

By default when the script is ran it will run the `main` function which will run everything the same way the script runs now.

It is now possible to do run `bin/build-release.sh package` to run just the final `tar`ing phase of the build.

I've also added a new function `downloadStormRelease` that can be used to download the storm zip release.