Travis CI: Run flake8 tests to find Python syntax errors and undefined names
Travis CI: Run flake8 tests to find Python syntax errors and undefined names

Add [flake8](http://flake8.pycqa.org) tests to find Python syntax errors and undefined names.

__E901,E999,F821,F822,F823__ are the "_showstopper_" flake8 issues that can halt the runtime with a SyntaxError, NameError, etc. Most other flake8 issues are merely "style violations" -- useful for readability but they do not effect runtime safety.
* F821: undefined name `name`
* F822: undefined name `name` in `__all__`
* F823: local variable name referenced before assignment
* E901: SyntaxError or IndentationError
* E999: SyntaxError -- failed to compile a file into an Abstract Syntax Tree
m-labs/pythonparser