set session to null after invalidating it
Copyright update.
Closes #395.
Closes #454.
Merge pull request #456 from itmm/master

Fixes NullPointerException on invalid sessions
Fixes #462.
Closes #310.
Fixed invalid redo when target changes from remote to local.
Added release notes. Small fixes.
Update version to 2.5.1 final from snapshot.
Update version to 2.5.2-SNAPSHOT