added functions and unit test
added models that use these functions
updated Stan reference manual
added agrad fwd and rev function definitions
added unit tests for auto-diff versions
undoing change to make/local that got picked up
added const to function arguments
Merge pull request #1039 from stan-dev/feature/issue-592-expose-isnan-isinf

Fixes #592. Feature/issue 592 expose isnan isinf