Feature/issue 1129 neg binomial 2 cdfs

#### Summary:
Fix issue #1129. Adds neg_binomial_2 CDFs.

#### Intended Effect:
 Adds neg_binomial_2 CDFs.

#### How to Verify:
Check code and unit tests.

#### Side Effects:
None.

#### Documentation:
Included.

#### Reviewer Suggestions:
Anyone.
This seems like it's going to give you different answers.  Did you test you get the correct answers for all of these different sizes against some independent implementation, like say the one from R?