IPython 2.0 support (closes #7)

Hi @rgbkrk,

I thought about it and I think we should go with this code, here are some important changes:

- supports IPython 2.0 notebook manager api
- pyrax has been replaced by direct use of open swift client
- rackspace authentication will work only with username/password but not with the api key
- openstack credentials are fetched from the environment by default, like openstack utils do
- updated documentation

I understand the long term plan is to switch to libcloud so we should avoid referring to specific APIs. What do you think of renaming SwiftNotebookManager to simply NotebookManager?