API: rename _prop_attributes to __finalize__ (in NDFrame)
     rename _propogate_attributes to _metadata (in NDFrame)
     change methods which directly pass metadata to use __finalize__

     These provide a simple mechanism for meta data propagation, which
     can be overriden in subclasses if desired
Merge pull request #5205 from jreback/meta

API: rename _prop_attributes to __finalize__ (in NDFrame)