make remarkup savvier about fragments in phriction links

Summary:
use PhutilURI class to get the slug and the fragment, normalize the slug, and then glue it back together with another PhutilURI

fixes https://github.com/facebook/phabricator/issues/228

Test Plan: had a few links like wiki fun and verified links generated were correct

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3894