Updating version/release notes for 2.4.0 release.

2.3.1 changing asterisks to dashes to be like the other entries.

Updating versions for 2.3.0 release

Updating versions/notes for 2.2.1 release.

Merge branch 'master' into 2.2.0

release 2.1.0

Merge pull request OpenMDAO#410 from JustinSGray/master

Tutorials for derivs of explicit and implicit comps

changing versions to 2.0.1

OpenMDAO 2.0.0 release version