added NaN test for log1p_exp
added NaN test for log1p_exp
added doc for log1p_exp
Merge branch 'develop' into feature/test-nan-log1p-exp
Merge branch 'develop' into feature/test-nan-log1p-exp
Merge pull request #984 from stan-dev/feature/test-nan-log1p-exp

Feature/test nan log1p exp