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.

(cherry picked from commit 58d4f6c8a5d9739dc2a3f26f116528457336f0d3)
Signed-off-by: Reynold Xin <rxin@apache.org>