modify tries to set read-only reversibility flag

This flag was made into a read-only property calculated from the upper
and lower bounds, so it should not be directly set anymore.
Merge pull request #51 from aebrahim/modify_reversible_bug

modify tries to set read-only reversibility flag
add doc pages explaining reactions and metabolites

Touched up rst exported from ipython notebooks, which are also included
examples set reversibility

reversibility should not be set, but is calculated from upper and lower
bounds
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)
Merge pull request #52 from aebrahim/more_docs

More docs