Catch keyboard interrupt + misc minor improvements

Hi Thomas,

This PR contains some improvements and a bugfix for psutil 2.0 which changed its API:
- Catch keyboard interrupt to clean up things: close the log file, draw the plot, kill the process if needed.
- Process status is now a method (changed with psutil 2.0). + Print elapsed time.
- Allow to use a float interval.
- Pep8