Packages - make Invoke optional.

Invoke isn't *really* optional for development but it *is* for testing. So in favor of even faster test virtualenv building, don't install Invoke. Tests can be run with `python -m armstrong.dev.dev_django test` (instead of `invoke test`).
The readme will be updated in a separate commit with a few different things, but in essence a dev environment will need `pip install tox invoke -r requirements\dev.txt`.