Nexson import

OK, I think NexsonReader is ready for use now in developing new MainRunner commands that obtain sources trees from Nexson files.

For a sample input file please see gist https://gist.github.com/jar398/5100472. Note that it has ottol ids, but they are garbage. We are waiting for Rick to integrate the new ottol into Phylografter, before we can put real ids into this file. If you want you can just use the labels (tax names) instead as a temporary hack.

Usual caveats about this being preliminary, poorly tested code, but I felt it was important to put something out there that people can start programming against.