New (highly!) experimental features using macros.
The ProductDistribution class has been updated to handle any non-zero number of distributions.
Added a line of clarity to the docstring, courtesy of Chris Granade.
Merge pull request #19 from ysanders/master

ProductDistribution can now handle any non-zero number of factors
Fixed docstrings in macro-generated models.
Merge pull request #18 from csferrie/feature-experimental

New (highly!) experimental features using macros.
Continued work on refactoring into DerivedModel.
Merge branch 'master' into feature-canonicalize