New option to rediscover devices from web interface

http://snag.gy/fKfbd.jpg

This simply resets the last_discovered table entry in devices to NULL so that when a discovery -h new runs (which is every 5 minutes by default) then the device will run through a discovery again.

You can disable this by setting $config['disable_clear_discovery'] = 1; in config.php

This relies on PR #299 so please don't merge this until then.