Pascal: make r=1 by default.

[kevindavisross]

Also, should we revisit Pascal/Negative Binomial naming convention? (Larry Leemis, who wrote the big distribution chart, uses Geometric and Pascal with support on 0, 1, 2, ..., and he doesn't use the name Negative Binomial at all.) We could add an argument to Geometric or NegativeBinomial like count_successes=True/False, but does that violate the principle that the only arguments of Distributions are their parameters?