Merge branch 'develop' of https://github.com/stan-dev/stan into develop
deprecated abs(real), added stan::math version of abs(real); updating manual in manual branch to avoid conflicts
Merge pull request #263 from stan-dev/feature/issue-257-abs-deprecate

Fixes #257.
Deprecate abs(real).
Merge branch 'develop' of https://github.com/stan-dev/stan into feature/issue-211-stan2-manual
more doc updates for 2.0