Allow certain paths to be excluded from session generation.

`Set-Cookie` headers do not interact well with many HTTP-caching
strategies. This changesets makes it easy to avoid generating
session Information and thus `Set-Cookie` headers for certain paths.

E.g.

    app = SessionMiddleware(app,
        cookie_key=COOKIE_KEY,
        ignore_paths='^/images/.*$')

This pull request does not include updates to the test code because I didn't find an obvious way to integrate it. But I use the code in production. (see https://cloud.google.com/files/Hudora.pdf#)