Call Kryo setReferences before calling user specified Kryo registrator.
Moved the Spark internal class registration for Kryo into an object, and added more classes (e.g. MapStatus, BlockManagerId) to the registration.
Merge pull request #157 from rxin/kryo

3 Kryo related changes.

1. Call Kryo setReferences before calling user specified Kryo registrator. This is done so the user specified registrator can override the default setting.

2. Register more internal classes (MapStatus, BlockManagerId).

3. Slightly refactored the internal class registration to allocate less memory.