Handle "long ago" and "a while from now" times

I ran into an issue where times more than a year and a half out weren't being humanized.

First I added tests to reproduce the bug:

![failing tests](https://cloud.githubusercontent.com/assets/1128849/2756181/e9c13eea-c972-11e3-9a15-40606bfff886.png)

Then I made the fix and tests now pass:

![passing tests](https://cloud.githubusercontent.com/assets/1128849/2756183/f5fb7f54-c972-11e3-9b9d-1180bd024c5a.png)