Allow refresh=0 to silence all iteration output.
Merge branch 'develop' into feature/issue-427
Merge branch 'develop' into feature/issue-427
Add unit test for refresh=0 and refresh>0
adding doc for do_print(). added unit test.
Merge pull request #526 from stan-dev/feature/issue-427

Fixes #427. Allow refresh=0 to silence all iteration output.