Adds get_media_by_shortcode endpoint, tests, and fixture.

The Instagram API allows media to retrieved by its shortcode, via [`/media/shortcode/`] (http://instagram.com/developer/endpoints/media/#get_media_by_shortcode). However, this gem currently does not provide a method to access this endpoint.

This pull request adds a method `media_shortcode` that allows users to make requests to this endpoint.
It also includes tests and a fixture for the feature.