sort plots by modify time? 

I think it would be more natural to display all plots in creation/modification order?  I think changing line 121 in kernel.py from 
```python
for fname in sorted(os.listdir(tmpdir)):
```
to
```python
for fname in sorted(os.listdir(tmpdir), key=os.path.getmtime)):
```
would do it.  Thoughts?
