sbml loads charges from notes

If the charge is specified in the notes field, it will be saved as
metabolite.charge (which is what happens for charges specified as sbml
attributes).
unit tests are less verbose
ensure metabolites are added even with bad charge

add unit test to ensure this behavior as well
update pickles - iJO1366 should now have charge

Due to the previous commits, the iJO1366 model should now have charges
specified for the metabolites due to the structure of the xml file from
which the model was based.
Merge pull request #38 from aebrahim/sbml_charge

sbml loads charges from notes