Initial refactoring of htmlnotebook.txt into 3 parts. 

@ivanov:
 Splits `docs/source/interactive/htmlnotebook.txt` into three pieces: `notebook.txt`, `working_remotely.txt` and `converting_notebooks.txt`.

I also rearranged somewhat what was left in `notebook.txt`.

The current text in `converting_notebooks.txt` on `nbconvert` is out of date. 
Once the new command line options are fixed, this will need to basically copy the help text from:

    $ ipython nbconvert