Added distribution neg_binomial_log_log
Added neg_binomial_log_rng
Added neg_binomial_log_log to manual
Change neg_binomial_log_log to neg_binomial_2_log_log
Merge branch 'develop' into feature/neg_bin
Minor fix for neg_binomial_2_log_log
Added neg_binomial_2_log
added rng for neg_binomial_2
fix neg_binomial_2 in function_signatures
Small improvements to neg_binomial_2
Expanding documentation for neg_binomial_2
Expanding documentation for neg_binomial_2 (forgot to finish sentence)
Fix typo in stan/prob/distributions/univariate/discrete.hpp
renaming test
adding tests to make sure c++ syntax is correct
Merge pull request #555 from stan-dev/feature/neg_bin

adds neg_binomial_2_log, neg_binomial_2_log_log