Allow dumping any BasicObject that defines #marshal_dump or #marshal_load

Additionally, it is assumed that these objects will have implemented
`respond_to?` and `class`, (e.g. SimpleDelegator). These assumptions are
reasonably inline with the assumptions of `Marshal` in regards to
`BasicObject`s.
Merge pull request #205 from sgrif/sg-simple-delegator

Allow dumping of Delegator subclasses