feat(client): add "--buildpack" option to "deis create"

Adds option "--buildpack" (and shortcut "-b") to the "deis create"
command. This allows setting the buildpack url when creating a new app
in the same manner as the "heroku" command line tool.
test(client): add --buildpack tests
style(client): flake8
Merge pull request #2720 from bacongobbler/create-with-buildpack

feat(client): add --buildpack option to `deis create`