brought backend_qt4.py into pep8 compliance.

Not confident that the change to line 399 is the correct thing to do
here.
brought ../backends/backend_qt4agg.py into pep8 compliance
brought qt4_compat.py into pep8 compliance
minor indentation fix
docstring formatting fix
Merge pull request #2153 from tacaswell/qt4_pep8

Qt4 pep8