[Issue #139] Fix time interval calculation involving weeks.

I'm not entirely sure if this resolves the issue in #139 but it's likely related. Personally, I was having trouble getting the weeks between two dates to be returned correctly. For example:

```objc
    now = [NSDate dateWithTimeIntervalSince1970:1396281600]; // April 1, 2014 00:00 UTC -08:00
    dateToTest = [NSDate dateWithTimeInterval:-604800 sinceDate:now];
    output = [formatter stringForTimeIntervalFromDate:dateToTest referenceDate:now];
    // expected `output` to equal `1 week ago`, got `7 days ago` instead
```

This resolves that issue, and hopefully by extension the one referenced here as well.