allow third party hubs to be used with EVENTLET_HUB environment variable.

This change allows the use of third-party hubs with the `EVENTLET_HUB` environment variable, which greatly improves the ability to test those hubs against the eventlet testcases with nosetests as well as switching to those hubs without using `use_hub()` explicitly.

It adds the ability to do the following:

 - if there's a dot present in the hub name then it will attempt to import that module without prepending `eventlet.hubs`:

    `$ EVENTLET_HUB=somepackage.hubmodule nosetests`

 - if there is a colon it uses the package.subpackage.module:Class convention to import the hub class:

    `$ EVENTLET_HUB=hubmodule:Hub nosetests`

 - and finally, extending the simple module syntax, it will look up name in a setuptools/distribute entry point group 'eventlet.hubs' (though this is only used if the setuptools/distribute provided `pkg_resources` module is found, it doesn't depend on it being there):

    `$ EVENTLET_HUB=name nosetests`

which can be registered by module with the following in their setup.py:

    entry_points = {'eventlet.hubs': 'name = somepackage.hubmodule:Hub', }

and if that fails, it imports the hub using `eventlet.hubs.%(name)s` as before...