authorize:Authorize: Add deny & regex flags.

This patch adds two new features to the authorize:Authorize filter:

- Add flag to change the policy from default-deny to default-allow. This
  makes it possible to block specific users.
- Add a flag that changes the matching algorithm from regex matching to
  string matching, which simplifies matching with strings that contain
  special characters.

This patch also makes it simpler to subclass this filter.

Thanks to Ryan Panning for implementing this!

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2987 44740490-163a-0410-bde0-09ae8108e29a