added NaN test for acos
update nan test for agrad/rev/acos
added NaN test for agrad/fwd/acos
switched to no namespace version of acos to match double call
added bound tests for acos
added NaN doc and derivative doc for acos
updated fwd/matrix tests
Merge pull request #947 from stan-dev/feature/test-nan-acos

Feature/test nan acos