Merge branch 'develop' of https://github.com/stan-dev/stan into develop
Merge branch 'develop' of https://github.com/stan-dev/stan into develop
updated release notes for 2.3
updating makefile message for test-models
Merge pull request #702 from stan-dev/feature/issue-698-make-error

Fixes #698. updating makefile message for test-models
adding stan-reference-2.3.0.pdf to repository for v2.3.0
updating makefile with clearer language
Merge branch 'develop' into feature/issue-690-tag-v2.3.0

Conflicts:
	makefile
Merge pull request #703 from stan-dev/feature/issue-690-tag-v2.3.0

Tagging v2.3.0. Fixes #690.
Merge branch 'develop' into feature/issue-676-generalize-operandsandpartials