Fixed declaration of scaling parameters
NBD sites to fit are passed into Builder constructor
Added a random_initial_values method to core.Builder

Random initial values are based on the prior distributions
for parameters specified in the model building functions.
Currently just samples from the priors.
Added some safety code to make sure NBD is set

...and deleted some old model building code
Fixed: random_initial_values returns linear, not log scale values

Because the values are fed directly into the MCMC object, and since
the MCMC object does its own conversion to log10 scale, these should
be in linear scale.
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.