Made organize imports compatible with OpenStack standards

This adds two options that can be used to make 'organize imports'
compatible with the OpenStack hacking standards
(http://docs.openstack.org/developer/hacking/#imports), specifically
H301 and H306.  (Some of the other standards can already be applied
with rope, like H303, while others, like H302, are well beyond the
scope of this change.)

The two options are added to default_config.py, and documented
there.
Split imports in AddingVisitor

This makes organizing imports better able to remove duplicate imports
with splitting enabled.
clarify splitting/duplicate logic when organizing imports
Merge pull request #66 from stpierre/organize-imports-for-openstack

Made organize imports compatible with OpenStack standards