Fix flake8 errors and enable in travis

Clean up whitespace, extra imports, redefined symbols,
and other errors and warnings reported by flake8.

Add flake8 to the commands run by travis.

Change-Id: Id3bdea24cad61c13c163970a4a13bc47e4ece553
Merge pull request #58 from dhellmann/flake8-fixes

Fix flake8 errors and enable in travis