adding cell navigation (up, down)

[skariel]

in hotkeys.js this does the job if you insert them at the beginning of the document_keydown function (so pgup/down are not consumed by goto top/bottom of page):

    if (event.which == key.PGUP && event.altKey) {
        IPython.notebook.select_prev();
        IPython.notebook.scroll_to_cell(IPython.notebook.get_selected_cell());
        return false;
    };

    if (event.which == key.PGDOWN && event.altKey) {
        IPython.notebook.select_next();
        IPython.notebook.scroll_to_cell(IPython.notebook.get_selected_cell());
        return false;
    };

[juhasch]

Looks good to me, I have added this. Thank you.

[juhasch]

Changes commited.