checkfiles: git hook to run autopep8 + automatic import reordering

usage:

for one file:
echo master/buildbot/process/buildstep.py | common/checkfiles
for all files:
find . -name \*.py | common/checkfiles

as a git hook:
cp common/hooks/post-commit .git/hooks

After you do your commit, the script will run on the modified files
You see the results in the terminal, you can review it, and amend your patch

Change-Id: I2f229065c151d40d10a45fa83da44a77c793cc08
Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>