Option for messages to not fade-out automatically

I've implemented a new feature to allow messages to remain onscreen, aka 'sticky' messages. This can be configured site-wide from the purr messages admin screen or alternatively called on a per-message basis by specifying the type as 'sticky'. See http://tanc.me/purr-messages-demo for a demonstration.

Example code for displaying a sticky error message if site wide sticky messages are not turned on:
drupal_set_message('This is a <strong>sticky</strong> message which is also an <strong>error</strong> and will only clear with user interaction', 'sticky error');