Fix computation of DoctestSyntaxError.lineno and col
Add boolean attribute Checker.withDoctest to ignore doctests
Add environment variables PYFLAKES_NODOCTEST and PYFLAKES_BUILTINS
Add tests for DoctestSyntaxError lineno and col
More tests for doctest (lineno, col) calculation
Fix regression with Pyflakes 0.7, and new variable to disable checking doctests