Provide a mechanism for reverisbly decoding arbitrary data for infix URIs

Summary:
See next diff. In Diffusion, we want to put arbitrary data into the URI and keep it as readable as possible.

Correct phutil_escape_uri()'s handling of slashes for suffix or query parameter use.

Add phutil_escape_uri_path_component() to correctly handle arbitary URI infixes (notably, branch names).

Test Plan: See next diff.

Reviewers: nh, vrana, btrahan

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1920