Some more OSGI improvements

- Export "internal" impl packages, needed for serialization
- Make default ExpressionFactory instantiation work in OSGI:
  - Try both TCCL and API bundle classloader (before: try only TCCL if available)
  - Add optional import of implementation package in API package
    ( this is not a very nice thing to do - but should work,
      alterative: make impl a fragment of api )