updating version from 1.3.0 to v2.0.0
working on downloads
Merge branch 'develop' into feature/issue-246-website
Merge branch 'develop' of https://github.com/stan-dev/stan into feature/issue-246-website
Merge branch 'develop' of https://github.com/stan-dev/stan into feature/issue-246-website
updated web site
Merge pull request #282 from stan-dev/feature/issue-246-website

Fixes #246.