[ATMO-225] Change to add more to version.
	modified:   atmosphere/version.py
Initial step implementation.

Implemented the serializer and api for step. It was templated from size and not complete.
Reworked step task to work.
Renamed version datetime to date.
	modified:   api/serializers.py
	modified:   api/step.py
	modified:   atmosphere/version.py
	modified:   core/models/step.py
	modified:   service/__init__.py
	modified:   service/deploy.py
	modified:   service/task.py
	modified:   service/tasks/step.py
Merge branch 'master' of github.com:iPlantCollaborativeOpenSource/atmosphere
Make ipdb an optional requirement.
	modified:   service/tasks/machine.py
Re-order requirements so dev requirements are explicit.
	modified:   requirements.txt