OAK-625 : Ability to pass workspace name to a CommitHook (wip)

- move definition of default workspacename completely to OAK.java
- add test for Oak#with(String defaultWorkspaceName)


git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1446162 13f79535-47bb-0310-9956-ffa450edef68