Make it possible to specify Java classes as mappers, reducers, partitioners, etc.

[iconara]

Currently there's no way to mix Ruby and Java classes in a job. Sometimes you want to write the mapper in Ruby and use a built-in reducer, or a custom partitioner written in Java. This is not possible (except it you do a raw set, but that isn't very pretty).

[iconara]

I think this was fixed by #27

[grddev]

Actually, #27 only fixes setting the {map_,}output_{key,value} classes. More code is required to support Java mappers, reducers and whatnot.