Release docs to a gh-pages branch

As part of the `build:and_release_if_updated` task also build a static
version of the templates with a couple of minor variants. This will let
us show people what the template looks like in its barest form.

Push these rendered templates to the gh-pages branch which will be
publicly viewable on github.com.

It also copies in a copy of all the pkg tar files so that people can
download a version of the compiled templates.
Also produce a tarball of mustache version
Checkout branch at clone and don't escape string

- Checkout the correct branch at clone rather than running an extra
  shell command to switch branch
- Remove shellescape as `FileUtils` will escape by default and this
  would double escape causing failures
Publish tarballs to github rather than commiting

Add a new publisher which creates a release on github and uploads a copy
of each of the tarballs from the pkg folder. This will need an
`GITHUB_TOKEN` env variable to be set which should have write permission
to the govuk_template repo.
Add a nice name to the release
Merge pull request #108 from alphagov/release-docs

Release docs to a gh-pages branch