OAK-608 : Pluggable Authentication (WIP)

- refactor token provider to simplify pluggability
- add CompositeTokenProvider that allows to aggregate multiple tokenprovider implementations

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1528518 13f79535-47bb-0310-9956-ffa450edef68