Add flake8 checks to the test suite

This changeset adds `flake8` to the project test dependencies and
executes checks before the Django test suite. A failed `flake8` run
emits a known string ("flake8 check failed") so that the Jenkins text
finder plugin can mark the build unstable.

Attempts to resolve #70.
Merge branch 'feature/hmc/flake8' into develop

Conflicts:
	src/nyc_trees/requirements/test.txt