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!
Yeah, let me have a go at the `CreatedAt()` and `LastModified()`. Can I try a `CreatedAt(operator String, target time.Time)` which can take the forms of:
```go
.CreatedAt(">", time.Now() - time.Day) // Include only files that are 1 day old
.CreatedAt("<", time.Now() - time.Week) // Include only files older than a week
```