make build script pep8 conform

I hate that my pep8 checker goes crazy everytime I open one of them
revert changes to 3rd party files

protoc.py, qt*.py should be handled by upstream
Fix tooltip in key preferences.
fix errors from autopep8
Rename DlgPrefPlaylist to DlgPrefLibrary.
convert tabs to spaces
Merge pull request #107 from kain88-de/tabs

convert tabs to spaces
Merge pull request #106 from kain88-de/pep8-scons

make build script pep8 conform
tabs to spaces also in cpp files
Strip \r from all src/ files.
Only create one QApplication (in GUI-mode) per mixxx-test run.
Minor optimization to BeatGrid::findNthBeat.