adding src/stan/math/functions.hpp
rearranged stan/agrad/rev/ to match stan/math/
rearranged stan/agrad/fwd/ to match stan/math/
cleaned up top-level stan/agrad/*.hpp includes
Merge branch 'develop' into feature/issue-443-reorganize-agrad

cessary,
Merge branch 'develop' into feature/issue-443-reorganize-agrad

Conflicts:
	src/stan/agrad/rev/matrix.hpp
Merge pull request #447 from stan-dev/feature/issue-443-reorganize-agrad

Fixes #443.