incrementing version number to 2.1.0
adding artifacts (manual + api doc) to the repository for v2.1.0
Revert "adding artifacts (manual + api doc) to the repository for v2.1.0"

This reverts commit cb8a91067d40ed64793aa88ad3ffefadbe665ce2.
Merge pull request #466 from stan-dev/release/v2.1.0

release/v2.1.0
Merge branch 'develop' of https://github.com/stan-dev/stan into feature/stan-language-paper
updated first half of paper