Change tox.ini style with multi-line settings

Prefer to indent to a consistent width (four spaces) and always
breaking the line after the '=' sign for multi-line settings.
Add pep8 and pyflakes checking and omission criteria

These settings are the ones used so far to vet WAL-E by me.  It seems
to add an inconsequential amount of run time to the test suite, and
will hopefully help contributors submit patches less laborious for
those who maintain this once-arcane and unwritten standard.