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.