Add exercise conventions
Add exercise conventions

closes #478
exercism/xpython,N-Parsons/exercism-python,mweb/python,N-Parsons/exercism-python,smalley/python,pheanex/xpython,exercism/python,pheanex/xpython,exercism/python,jmluy/xpython,smalley/python,behrtam/xpython,mweb/python,exercism/xpython,behrtam/xpython,jmluy/xpython