move pFBA from oven to mainline
fix revert_to_reversible rxn/met relationship

need to use model.remove_reactions to make sure reaction/metabolite
awareness is updated
relative imports in parsimonious.py

now that it has been moved out of the oven
unit tests and bugfixes for pFBA/make_reversible
Merge pull request #61 from aebrahim/pFBA

Move pFBA from oven to mainline