Feature/issue 860 erf nan

#### Summary:
Fixes issue #860. 

#### Intended Effect:
agrad/rev/erf should now return NaN for gradients and value when passed in NaN.

#### How to Verify:
Run test/unit-agrad-rev/functions/erf

#### Side Effects:
None.

#### Documentation:
None. Should document behavior of function when given NaN together with other functions.

#### Reviewer Suggestions:
Anyone
std::sqrt(pi() * 0.5) * - INV_SQRT_TWO_PI

(should we just add a constant?)