Big rewrite of purifier module.
- move config from separate file to static variable in class (cascading config
files aren't too useful when 0 other modules are loaded).
- rename Purifier::purify() to Purifier::clean_html().
- add Purifier::add_config_group() to override settings if desired.
- add Purifier::clean_input_array() to clean GPC superglobals.
- add module.info (v3 to avoid confusion with Gallery 3.0.x v2).
- add unit tests (not yet validated).
- update SafeString to reflect changes.
Add superglobal XSS cleaning code (and purifier module) to bootstrap.
Since we're clearing HTTP_HOST here, we no longer need our URL::base() override.