Don't report UnusedImport when binding global name

When an import binds to a name declared global by the "global"
statment, don't report it as unused.

Add tests to document hard to test cases

It would be nice if shadowing an unused variable was an error, but this
is quite difficult to implement in a way which does not also raise false
positives in the context of loops.

Avoid running flake8 under nightly

PyPI's pep8 is not compatible with Python 3.5 yet.

Shadowing a used variable should still potentially raise an
UnusedVariable warning. Alter tests' expected results to agree with
this premise.