Add help to factoids

I'm not a regex guru, and to figure out how to actually use the factoids I had to take the various regexes and put them into http://www.regexper.com/ just to figure out what the valid commands actually are. I think each plugin should come with some help text on how to use it.

I've a [pull request into the ircjsbot project](https://github.com/nlogax/ircjsbot/pull/5) that would add a help command to the bot which would look for each plugin to export a help string to use in a response.

For the factoid plugin I propose the following help text:

> Set factoids with "!factoids is 'factoids are great!'". Forget factoids with "!forget factoids". Retrieve with "!factoids" or "@factoids".

The command would then be:
```bash
help factoid
```

Code will follow as soon as this DDOS attack on GitHub stops.
![GitHub System Status 2013-03-23 17-27-22](https://f.cloud.github.com/assets/28543/294769/37a5684a-9411-11e2-91be-237c68fba28f.jpg)