Merge branch 'develop' of https://github.com/stan-dev/stan into develop
added deprecation warning for LKJ covariance with tests
clarified when lkj_cov would go away in messages
updated doc to remove lkj_cov distribution
Merge pull request #1051 from stan-dev/feature/issue-865-lkj-cov-deprecate

Fixes#865. Feature/issue 865 lkj cov deprecate