Fix stan::math::exp behaviour for NaN (for matrices of doubles)
Using iterators instead (in stan::math::exp)
updating exp to call the unqualified one, which should be stan::agrad::exp
stan::math::exp(Eigen::Matrix) will have an specialized version for double that will check for nan inputs
even simpler and better solution
added NaN test for exp
Merge pull request #903 from stan-dev/feature/issue-902-exp-nan

Fix stan::math::exp behaviour for NaN (for matrices of doubles) 

Fixes #902