Feature/test nan exp2

#### Summary:
Added NaN tests for exp2 in agrad/fwd/functions, and agrad/rev/functions with doc on behavior at NaN. I also switched to the no namespace version of exp2 to match the double version.

#### Intended Effect:
Added NaN tests for exp2 in agrad/fwd/functions, and agrad/rev/functions with doc on behavior at NaN. 

#### How to Verify:
Run test/unit-agrad-fwd/functions/exp2, test/unit-agrad-rev/functions/exp2

#### Side Effects:
None.

#### Documentation:
Added doc on behavior of exp2 at NaN and partial derivatives.

#### Reviewer Suggestions:
Anyone.