Document Loader#load
Add load_marshal_data to handle unloadable data

When using DRb::Message to load objects from the DRb message stream
DRbUnknown objects were created automatically.  After switching to
DRbDump::Loader unknown objects would result in an exception.

This new method creates DRbUnknown objects like DRb::Message did.
Add example/service_primes.rb for analysis

This is an interesting alternative to the tuplespace-based prime
generator example due to the greater variety of messages sent.
Rename primes.rb to tuplespace_primes.rb