Starting to break out the artifact uploading functionality

Signed-off-by: Rachel Thompson <r.thompson@modcloth.com>
Filling in tests for the directory upload case
Using `upload-artifact` inside the `upload-go-artifacts` command/func
Documentation brushup for `upload-artifact` command [ci skip]
Like, duh, authors!
Merge remote-tracking branch 'origin/master' into break-out-artifact-upload
Using the new `$runner.unstub` interface
Merge remote-tracking branch 'origin/master' into break-out-artifact-upload
Merge pull request #2 from modcloth-labs/break-out-artifact-upload

Break out artifact upload