added failing NaN test for fmin
fixed agrad/rev/fmin so gradients return NaN when input is NaN
added failing NaN test for agrad/fwd/fmin
fixed agrad/fwd/fmin so derivatives return nan for nan input
added doc for fmin
Merge branch 'develop' into feature/test-nan-fmin
Merge branch 'develop' into feature/test-nan-fmin
Merge pull request #1000 from stan-dev/feature/test-nan-fmin

Feature/test nan fmin