Add /stars/recent endpoint

This endpoint can be used to export JSON or CSV data for stars given
during a certain timeframe. The result is ordered by number of stars and
grouped by the recipient(s). By default, stars for the last 30 days will
be returned. You can change that by passing in `Date#parse`:able strings
in `to` and `from` params, e.g.

/stars/recent?from=2014-02-27&to=2014-03-27

This will give you stars from February 27 to March 27.