Add support for relative units: "yesterday"

This adds support for selecting common used relative units for each locale; e.g., instead of outputting "1 day ago" these changes will now output "yesterday".

This also adds support for a `units` option, to _always_ force the output to be in the specified units, the following example forces the output into hours:

```js
var rf = new IntlRelativeFormat('en', {units: 'hour'});

// Generate a timestamp that's two days ago.
var output = rf.format(Date.now() - (2 * 24 * 60 * 60 * 1000));

// Outputs hours _instead_ of "2 days ago".
console.log(output); // => "48 hours ago"
```
Not sold on this name, but what do you call "now", "tomorrow", "yesterday", "next month", etc.? I'm open to better ways of referring to these. In the CLDR data they are inside the `relative` collection.