applying nested auto-diff changes from ode branch
added text fixtures and explicit memory recovery in unit-agrad-fwd tests that needed it
fixed broken tests from new autodiff nesting and moving dependencies
added exceptions for uses of recover_memory not meeting nesting requirements plus tests
Merge pull request #1033 from stan-dev/feature/issue-1031-nested-reverse-mode-autodiff

Fixes #1031. Feature/issue 1031 nested reverse mode autodiff