When ignore is an empty string, it will apparently not run any checks at
all. Using None as the default ensures that it does run the appropriate
checks.

Closes #52

os.path.relpath on Python 2.6 returns slightly different output than it
does on 2.7. Rather than try to write tests around the behaviour, it
makes sense to have a common relpath function that behaves the same on
every version.

Conflicts:
	flake8/hooks.py

The simpler fix is to ensure that the filename isn't the full path to
the temporary file before we check to see if it should be excluded or if
it should even match.

This also fixes a bug I found while testing the pull request in which no
files are checked.

Get parameter's value using git config before trying os.environ.