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- pyrax has been replaced by direct use of open swift client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?

I'm not sure I understand what you mean. This is `notebook_exists` which is supposed to “Returns a True if the notebook exists. Else, returns False”. I changed the request from a GET to HEAD to make it less expensive, as we are only checking for the object existence. Am I missing something?