Replaces new_notebook method in Notebook and NotebookList classes

Replaces new_notebook method in Notebook and NotebookList classes with ContentManager.new_notebook.

This PR also contains some style/typo fixes, making ContentManger look more like other IPython files.

I also make ContentManager stateless, in that the only state is base_url.  The reason I prefer this is that if ContentManager is responsible for maintaing state such as the current notebook's path and filename, then we will have to replicate this logic in every implementation of ContentManager.  Similarly, I don't think that save_notebook and other functions should know about the Notebook object.  They should just take JSON and a path/filename.  How does that sound to you?

So far the only changes in the code in this direction are in the new_notebook method and the constructor.