Serialization fixes

[johnynek]

This does three things:

1. Moves serialization to a new package: scalding.serialization
2. Doesn't use Kryo for writing classes in collections (most questionable, but we are seeing some issues with unregistered classes).
3. Make sure to flush after each object write in collections (important to avoid bloating the intermediate buffers).

[travisbot]

This pull request fails (merged b1b32b1 into 73d0450).

[travisbot]

This pull request passes (merged 59e91eb into 73d0450).

[travisbot]

This pull request fails (merged 999c377 into 73d0450).

[travisbot]

This pull request fails (merged 77cf4da into 73d0450).

[travisbot]

This pull request fails (merged b12c9aa into 73d0450).

[travisbot]

This pull request fails (merged 00b9332 into 73d0450).

[travisbot]

This pull request fails (merged dd4285a into 73d0450).