small fix for beta distribution error messages
Added neg_binomial_2_cdf + tests
Added neg_binomial_2_cdf_log + tests
Added neg_binomial_2_ccdf_log + tests
added neg_binomial_2 cdfs to functions signature and syntax tests
added neg_binomial_2 cdfs to manual
Improvements and fixes to neg_binomial_2 cdfs
More tests for neg_binomial_2 cdfs
missing using statement
Merge pull request #1140 from stan-dev/feature/issue-1129-neg_binomial_2-cdfs

Feature/issue 1129 neg binomial 2 cdfs