neg_binomial_log_log

#### Summary:

Adds neg_binomial_log_log and neg_binomial_log_rng.

#### Intended Effect:

Help with GLM negative binomial regression. See thread:
https://groups.google.com/d/topic/stan-dev/BwlOKOZ3uxg/discussion

#### How to Verify:

unit-distribution tests added

#### Side Effects:

No.

#### Documentation:

Documentation about the distribution and rng added to the manual in the usual way.

#### Reviewer Suggestions:

Bob Carpenter