added a new powerlaw model with a test
Old commits:
* fix models reference in test and use super to init
* needed to add powerlaw to __all__
* need to use parameters.Parameter
* power law model only takes 2 pars
* tests still fail, but the error no longer makes any sense
* fixed the tests.  Sigh.
* skip test if no scipy
* use the derivative!
* corrected deriv function (was correct at some time earlier; must have
forgotten to pull...)
Add a docstring to PowerLawModel
Minor changes to finalize power law
Merge pull request #1184 from nden/powerlaw-model

Powerlaw model