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.
If phi is small compared to n, the cdf will be 1.

I've done this ugly hack because otherwise, the upper bound test wouldn't pass, since if n is largest long int 2147483647, then n + 1 (that is passed to beta_cdf) will be -2147483648!

But, how is the correct way to deal with that?