added error_checks for univariate continuous distribution RNGs
added error checks for univariate/discrete RNGs
added error checks for multivariate/continuous RNGs
added multivariate/discrete error_checks for RNGs
fixed error in normal distr
fixed neg_binom_rng
Merge pull request #194 from stan-dev/feature/fix_rng

Fixes #188.
Feature/fix rng