Add time of execution start

[Benjamin-Loison]

For instance in:

>>> (executing cell "" (line 1 of "test.py"))

As I often have Pyzo running in the background but I do not remember when was the last time I executed a code (to avoid running it again as it takes a bit of time and would be useless if it was already executed a few minutes ago) that I sometimes execute. The code I have in mind is Travian sum_resources.py.