Wholesale rewrite of NBD fitting to pysb models

* Likelihood function is generated from a closure, taking nbd residue as argument
* Arguments for the model, nsteps, nbd site, etc. can be passed in as
  command-line arguments.
* Figure plotting, simulations and normalizations, etc. are separated
  out as functions

Bottom line is now the model to be used and the NBD data to fit can
now be passed in as arguments, facilitating more systematic evaluation
of fits of the various models to data.