Explicitly set 'interfaces/python' as the first entry in sys.path, rather
than reyling on the PYTHONPATH environment variable, which is sometimes
superseded, e.g. by virtualenv.
For reactions with unity reactant stoichiometric coefficients, explicit values
for the forward reaction order were being ignored while setting up the
StoichManager.
Added a few tests that confirm that these rates are being calculated correctly.
