various unicode / url fixes

The problem:

we had a variety of ways to get the notebook name, path, and base url:

- `$('body').data("notebookPath")`
- `various_objects.notebookPath()`
- `IPython.notebook.notebook_path`

and there was a mixture of encoded and decoded URLs, causing incorrect URL errors with spaces or unicode filenames.

This should hopefully clean up how we work with URLs by eliminating some of the inconsistency:

- remove notebookPath, notebookName, and baseProjectUrl methods everywhere
- use notebook_path *attributes* instead
- we should never store encoded URLs, only store decoded text
- perform encoding only when making an actual request
- base_project_url is changed to base_url