DSL should warn/error when setting output_fields for java spouts

[dgolombek]

The DSL happily pretends to set output_fields for spouts implemented in java, but then drops them on the floor (https://github.com/colinsurprenant/redstorm/blob/master/lib%2Fred_storm%2Fdsl%2Ftopology.rb#L48). We can either try to fix this correctly (check if the spout implements ISchemableSpout or IMultiSchemableSpout and if so set the scheme via it, otherwise give up and raise) or we can just give up and raise an error (which would make #99 even more important to fix). Let me know what you think (or if I'm missing something) and I can submit a patch.