session443 6.x-1.0-alpha3

Total rewrite:

- Removed previous method of instant redirects. Instituted a method where
anything can indicate a preference during page generation, and any
preferences are acted upon before the page is rendered.

- Make it easier to disable the redirection by having a setting, rather than
being always on. This makes it possible to disable even if Drush is not
present.

- No longer necessary to edit settings.php

- Removed previous method of using a custom access-denied page. Now we just
check HTTP headers before the page is rendered.