import from _backend_bases in backend_agg to avoid implicit Gcf import
pep8 on backend_agg.py
split backend_gtk3agg.py into two parts, the gcf dependent and
gcf independent parts.

Same poor naming scheme as everything else in this branch.
updated coding standards exclude list
pep8 on backend_gtk3cairo.py
pep8 on backend_cairo.py
changed backend_cairo.py to import from _backend_bases to avoid
Gcf
updated coding standards
Split backend_gtk3cairo.py into two parts, one dependent on Gcf
and one not.

Same naming scheme as rest.