automatically re-run cell if content changed

[ihrke]

I think it would be convenient to re-run cells whose content has changed.
One way is to hash the cells' code and put it into the pickle file and compare it when re-running.
We could enable/disable this behavior with some configuration flag, e.g., --enable-auto-rerun.

[vascotenner]

At this moment cache function does not store the code yet. In the function definition are some traces that this is very easy, including the line:

code = cell if cell.endswith('\n') else cell+'\n'