Added shorcuts to split cell, merge cell above and merge cell below.

OK, I think that we *desperately* need a shortcut key to split cell function... and also it would be great to have the same for merge cell above and merge cell below.

The task of splitting cell is a common task when you write documents containing a lot of text... or when you are doing a presentation using slideshow preset in the CellToolBar. The same, although with a less frequency, happens with merge cell functions... but yet I think they deserve a shortcut.

This PR implements this key events, and I have assigned the following keys (available in a row):
* Ctrl-m 7 --- split cell
* Ctrl-m 8 --- merge cell above
* Ctrl-m 9 --- merge cell below

That's all...

Damián

pinging @ivanov who agreed with me in the SciPy sprint about the split shortcut... hehe... :+1: