tests(docs): add documentation tests to test-acceptance script

Also fixes a missing import in docs/docs_requirements.txt.
chore(*): remove fleet/systemd units

These are now maintained in deis/deisctl.
Merge pull request #1889 from mboersma/1888-make-docs-acceptance

tests(docs): add documentation tests to test-acceptance script
Merge pull request #1891 from carmstrong/remove_unit_files

chore(*): remove fleet/systemd units
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
chore(router): Updated nginx to 1.6.2

This fixes a security issue in 1.6.1 http://nginx.org/
docs(releases): update release procedure with deisctl tasks
Merge pull request #1897 from bacongobbler/contrib-docs

docs(contrib): update docs with deisctl
Merge pull request #1899 from mboersma/update-release-checklist

docs(releases): update release procedure with deisctl tasks
Merge pull request #1898 from johanneswuerbach/patch-5

chore(router): Updated nginx to 1.6.2
docs(CHANGELOG): update for v0.12.0
chore(release): update version to v0.12.0
feat(contrib/digitalocean): Use native CoreOS integration

Use the native CoreOS experience as recently added by DigitalOcean.

This rewrites the previous provisioning script and replaced the tugboat
dependency by the new docl tool dependency.
Docl was written specifically to support the operations we need.
Once a new DigitalOcean CLI comes out with support for the metadata
flag the use of Docl can be reexamined.
feat(userdata): add deisctl to the coreos install
Merge pull request #1903 from Xe/feat/userdata/deisctl-in-userdata

feat(userdata): add deisctl to the coreos install
chore(deis): bump version to 0.13.0-dev
Merge pull request #1908 from bacongobbler/bump-version

chore(deis): bump version to 0.13.0-dev