Document draper versions

[amiel]

Olson 0.2 supports older versions of draper (with model_class). master uses source_class instead.

[kristianfreeman]

Could we alias model_class to source_class if it's undefined? I'm not near a computer with IRB so I can't fool around with it, but something like (warning, pseudo-code inbound):

unless respond_to?(model_class)
  alias_method :model_class, :source_class
end

[amiel]

That should work