Add flake8 to Travis configuration.
Fix F841 issue in facebookoauth example.
Run flake8 on examples and main code separately.

The examples have multiple F401 issues that seem more like false
positives.
Fix syntax error that mangled flake8 commands.
Stop using deprecated use-mirrors option with pip.
Update get_posts example script for Python 3.
Add flake8 to Travis tests. Fix #180.