read only properties for Reaction (genes and mets)

Now users can view the metabolites and genes using reaction.genes and
reaction.metabolites, but these properties are read-only. The value is
still stored in the prefiexed reaction._genes and reaction._metabolites
properties.
Reaction has .reactants and .products properties
depracate Reaction.get_gene()
Merge pull request #53 from aebrahim/reaction_clean

Clean up Reaction class (continued)