Deprecations of assess_objective and introduction of cobra.flux_analysis.reaction.assess*

In response to https://github.com/opencobra/cobrapy/issues/34, I've deprecated the assess_objective function because it's not really capable of assessing a multi-reaction linear objective.  I've introduced the module: cobra.flux_analysis.reaction that can be used to assess whether a given model has:

 1) the capacity to absorb products of a reaction operating at, or above, a specified flux
 2) the capacity to produce precursors for a reaction operating at, or above, a specified flux