[openstack|network] Openstack Router Layer-3 Networking Extension

Work in Progress, not to be merged
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See http://docs.openstack.org/api/openstack-network/2.0/content/router_ext.html

Basic functionality is there, with working tests.
We're testing this stuff against OpenStack Grizzly, but this should be available in Folsom too I believe (unconfirmed).

Missing stuff so far, to be done before merging:

- [ ] Proper mocking all around.
- [ ] Proper documentation.
- [ ] More tests.
- [ ] create_network request may need to be updated to support some Router functionality.

I'd love to get some peer review before merging this stuff if possible, since I've been barely exposed to this stuff.