Feature/issue 674 overflow gamma p derivative

Sorry, I thought I submitted this a few months ago.  At least the branch actually exists and not a figment of my imagination...

#### Summary:

Explicit asymptotic behavior for the gamma_p derivative to avoid an infinite loop.

#### Intended Effect:

gamma_p derivatives do not loop for large values anymore.

#### How to Verify:

Evaluate gamma_p at a large input using an autodiff variable.

#### Side Effects:

None.

#### Documentation:

None. 

#### Reviewer Suggestions:

@syclik