Feature/test nan log 2

#### Summary:
Added NaN tests for log2 in math/functions, agrad/fwd/functions, and agrad/rev/functions with doc on behavior at NaN.

#### Intended Effect:
Added NaN tests for log2 in agrad/fwd/functions, and agrad/rev/functions with doc on behavior at NaN. 

#### How to Verify:
Run test/unit-agrad-fwd/functions/log2, test/unit-agrad-rev/functions/log2

#### Side Effects:
None.

#### Documentation:
Added doc on behavior of log2 at NaN and partial derivatives.

#### Reviewer Suggestions:
Anyone.