Change the git-hook ignore default to None

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

flake8/hooks.py

diff --git a/flake8/hooks.py b/flake8/hooks.py
index 659e5d4..2d1db8d 100644
--- a/flake8/hooks.py
+++ b/flake8/hooks.py
@@ -213,7 +213,7 @@
 #   git config flake8.complexity 10
 COMPLEXITY = get_git_param('FLAKE8_COMPLEXITY', 10)
 STRICT = get_git_param('FLAKE8_STRICT', False)
-IGNORE = get_git_param('FLAKE8_IGNORE')
+IGNORE = get_git_param('FLAKE8_IGNORE', None)
 LAZY = get_git_param('FLAKE8_LAZY', False)
 
 if __name__ == '__main__':