Add basic support for Jython 2.7b1

This allows NetworkX to be used from Jython 2.7b1, and closes issue #1026.

The most significant change I introduced is that `generators.atlas` is not imported anymore by default. One can import it manually if they think their interpreter can handle it. This is what was already described in the documentation, anyway, as the docstring for the module said: "Because of its size, this module is not imported by default.", so I don't think anyone will complain about it.

I updated the relevant tests to reflect the change, and set them up so that they are skipped if the testing is being run in Jython. I have to say that many other, unrelated tests still don't pass under Jython, but that's a separate issue.