Port psmetadata.py back to ete2

[gaurav]

One of the original reasons for writing psmetadata.py was to have something that was bug-to-bug compatible with pstastic.py in terms of reading and processing NeXML files. Given time constraints, I ended up rewriting it in DendroPy, but it might be useful to bring it back to ete so that any metadata visible to psmetadata.py is guaranteed to be stylizable from pstastic.py.

If anybody does attempt this heinous task, @jimallman has figured out that some NeXML attributes end up in e.nexml_node.anyAttributes_ -- if we could use this insight to figure out where ete stores its parse tree, that would be really awesome.