Fixes #1 - Ability to easily define useful durations

I added some code to enable users to do the following:

```ruby

include UsefulDuration

useful_duration :fortnights, 1209600

1.fortnight.ago   # => 2013-05-10 17:54:46 -0400
12.fortnights.ago # => 2012-12-07 16:55:11 -0500
```

This uses a bit of metaprogramming magic that I'm not 100% experienced with, so I'm happy to modify anything to convention. I also didn't change VERSION or the gemspec. Not sure what common practice is on that :)

Cool library, thanks!
Jordan