New feature: Cutoff time for applying inWords

I wanted to control when timeago applies inWords. Older dates should appear with their original representation, and newer dates, for instance newer than a week, should get the fuzzy timestamp.

This commit adds a setting, cutoff, that can be set to the number of milliseconds that marks the cutoff between using the original and the fuzzy date.

I've tried to use your coding style, and passing tests are included, even if this is the first time I've written Javascript unit tests. Hope you can use it!

-martin