Addresses problems with `find_MAP` and discrete stochastic variables

Addresses #396. This pull request addresses some issues with finding Maximum a Posterior(MAP) estimates when a model contains discrete stochastics.

1. Provides a better performing minimization function (`fmin_powell`) when discrete stochastics are used.
2. If no `fmin` is provided and `vars` contains discrete stochastics, provide a warning, and switch minimization methods
3. Floor discrete stochastic values before they are returned so they match the support of the original distribution
4. Clean up >80 error message for PEP8 compliance.