feat(tests): add test-nightly.sh script for CI

To be used by our nightly buildpack / Dockerfile regression testing
jobs at http://ci.deis.io/view/example-apps/.
docs(contrib): update docs with deisctl
Merge pull request #1894 from mboersma/1893-nightly-tests

feat(tests): add test-nightly.sh script for CI
fix(tests): make test-nightly.sh executable
Merge pull request #1897 from bacongobbler/contrib-docs

docs(contrib): update docs with deisctl