Merge branch 'release/v2.0.0' into develop
removing stan-reference-2.0.0.pdf and doxygen from develop.
Merge pull request #290 from stan-dev/feature/remove_artifacts

removing stan-reference-2.0.0.pdf and doxygen from develop
fixed generated code, updated the call to finite_diff_grad from src/stan/model/util.hpp, added unit test for finite_diff_grad
changed the test_grad call in util to match the test_grad call in v1.3
Merge pull request #302 from stan-dev/hotfix/issue-295-diagnose_not_working

Fixes #295.