remove interpretPriors and prior function generators (#15)
update generatePars to use getPriorFuncs to generate deviates from a pdf

Use the .rvs method built into each of the scipy.stats functions
returned by getPriorFuncs to generate a random deviate from the prior
PDF. Since there isn't a dirac delta function, "fixed" priors need to
be handled separately.

Also remove the shearOpt option in generatePars since it wasn't being
used and had a galsim dependency.
remove import galsim from ensemble.py since it is no longer used
update format of prior lists in Model class. Closes #15

replace old style with mix of tuples and lists with new format
(string, args) for each prior.

Also make model A in line with model B using log normal vcirc rather
than linear.