[NEW] Adding basic activity stream plugin

git-svn-id: file:///var/local/code.tiki.org/tikisvn/trunk@46591 b456876b-0849-0410-b77d-98878d47e9d5
[ENH] Refactoring to allow custom events to expose their relations to other events (to display in event graph)

git-svn-id: file:///var/local/code.tiki.org/tikisvn/trunk@46592 b456876b-0849-0410-b77d-98878d47e9d5