Fixing #4055 Added keyboard shortcuts to some menubar items

Trying to close https://github.com/ipython/ipython/issues/4055.

This is still in progress; I need design feedback from people here.

I know I shouldnt include any css in `override.css`. I'm doing this because im getting less
compile errors; see https://github.com/ipython/ipython/issues/5632. That is a separate issue with my local dev environment. 

Some screenshots: 
![cell_keyboard_shortcuts](https://cloud.githubusercontent.com/assets/954858/2715815/8f324e5c-c516-11e3-8de6-e1dc1fdb3b99.png)

![file_keyboard_shortcuts](https://cloud.githubusercontent.com/assets/954858/2715809/889c061e-c516-11e3-9bf5-f5127766c5a3.png)

![insert_keyboard_shortcuts](https://cloud.githubusercontent.com/assets/954858/2715810/88a0046c-c516-11e3-88d6-d1a7c5c199ba.png)

![kernel_keyboard_shortcuts](https://cloud.githubusercontent.com/assets/954858/2715811/88a42222-c516-11e3-9b6a-2360a26cf7fe.png)