Make sure that Checker does indeed ignore the names given in the "pyflakes_ignore" setting.

PyFlakes' Checker wasn't honoring the "pyflakes_ignore" setting. Instead of modifying a global, use Checker's "builtins" parameter when calling its constructor.
Merge pull request #22 from OscarL/patch-1

Make sure that Checker does indeed ignore the names given in the "pyflakes_ignore" setting.