Added a new feature: Since

Hey, I've very new to go, and I'm finding this project an amazing resource to learn from.

I was going to use your project as an import to find some files based on their age, I thought I'd share the changes I've made thus far.

I'm not sure if I like the name i've given my function and it may be best to change it to MinDate()?

Also, for the tests to run you'll need to change the modification date on one of the fixtures to anything over 30 days ago (i.e 'touch -t 201301010000 fixture/f3/d1.yml'). Maybe do this within the tests (as a prepFixtures() func)?

My next step is to add another feature for finding symlinks if your interested. I'd like to hear your thoughts on how bad my coding skills are :smiley:

P.S. This is my first ever pull request!
There's nothing AFAIK :). My idea was also to integrate that, but with a different project like DateParser, and make this project depend on the other, but actually I had no time to do anything. If you prefer to keep it simple for the moment, you can just try to implement the ```CreatedAt(min, max)```. As soon as I find some time, I would like to refactor the code and make it a bit nicer (it was just to test the language). Thanks for your help, really appreciated!