Future times are failing (example test case).

Hi there, I happened to stumble on your library apparently an hour after you published it. :)

I noticed you plan to add support for future times, but looks like they're not functional yet. In case you didn't know, I wrote a simple test case.

```
$ go test .
--- FAIL: TestFormatReference (0.00 seconds)
	timeago_test.go:65: 29) FormatReference(2013-08-31 12:00:00 +0000 UTC,2013-08-30 12:00:00 +0000 UTC): expected in one day, actual in about a second
FAIL
FAIL	github.com/xeonx/timeago	0.032s
```