New Feature: Topic watchers

I've now implemented the ability for users to watch topics and be notified of replies.

For now, if you create a topic or reply to one, you are automatically subscribed to updates. If you wish to stop receiving updates, you can click the 'Unwatch' button next to the reply button on the topic page.

You can choose to watch a topic without replying, this is done by clicking the 'Watch' button next to the Add Reply button on the topic screen.

I intend to make it possible for you to reply to a topic without also subscribing but this was just the first step in a series of improvements to the notification system that I intend to make :)

As a small aside: I have now started to tag ForumSpark with version numbers to make Changelogs easier to see.