Fixed #101. Filled in _check_plan_workflow(..) logic.

The logic follows the description in #101. Added a few tests to
test_api.py expecting failure.
Fixed #103. Refactored test_api.py into multiple files.
Forgot to import make_response.

This ticket also relates to #103. By now we should be able to run all
the tests.
Merge branch 'issue102' of https://github.com/yeukhon/minion-backend into yeukhon-issue102

Conflicts:
	tests/functional/test_api.py