Add Makefile target for running pep8 and pep8 configuration

Currently pep8 configured to ignore ALL errors that occures in Buildbot
code.
Run pep8 on Travis
Fix pep8 issue E101: indentation contains mixed spaces and tabs
Check only master and slave subdirectories with pep8

www/ subdirectory contains third party Python scripts which coding style
is not interesting to us.
Fix pep8 error E111: indentation is not a multiple of four

File master/contrib/buildbot_json.py was completely excluded from pep8
analysis because it uses two spaces for indentation.
Merge branch 'use_pep8' of git://github.com/vrutsky/buildbot
add pep8 to validate