This adds support for retrieving the tarball or zip source artifact from a
Github release. This is done through defining a
"params.include_source_tarball" or "params.include_source_zip" boolean setting.
With this, the resource will download the respective source artfact into the
target directory as either "source.tar.gz" or "source.zip".
google github library returns *Foo in singular case and []Foo if there
are many results. seems like the best approach is to pass them around
by-value, since *Foo seems to only be so that they can return 'nil'
instead of a zero-value.
