add a spec for namespaced treebank files

when reviewing the treebank retriever/persister code I was curious to see if it handled namespace prefixed treebank files, so I decided to use it as an opportunity to add a spec for this (happy to see that it passed).  In the process added the ability for aU.map to apply arguments to the callback.

(I think I messed up the branching though because it also contains a change from my bma_comments branch already submitted in a separate pull request)