Execute timings initial commit

This extension displays below the input box when the last execution of
a cell occurred and how long it took.
updated the license and the description
Added toggle all/current menu

Also got rid of checkboxes in the cell toolbars
Starting the timer on a more precise event

Patching cell.execute() to trigger a new event. Listening this event
instead of execution_request.Kernel
Merge pull request #77 from jcjaskula/executeTimings

Show when and how long a cell has been executed