Before this patch, executing directly one of these scripts, for example
`./plot_hystogram.py` in a unix-like environment meant that the default
system-level python would be used, regardless of an eventual activated
virtualenv.
This was due to the "#!/usr/bin/python" shebang.
Changing it to "/usr/bin/env python" is a fairly standard practice, keeps intact
the compatiblity with the system level python, and allows a user to run in a
virtualenv if he wants.
David Peter