Restrict which classes can be deserialized

To avoid a recently reported Java deserialization vulnerability [1], we should restrict which classes are accepted when deserializing binaries.

I have created a prototype SafeObjectInputStream at [2], which refuses to operate on classes that are outside a whitelist.

We probably also need a wrapper for ObjectInputStreams provided by the environment, that looks a bit harder to create, for now we can already discuss this prototype to see if we want to pursue the idea.

[1] https://blogs.apache.org/foundation/entry/apache_commons_statement_to_widespread

[2] https://svn.apache.org/repos/asf/sling/whiteboard/bdelacretaz/safe-object-input-stream