phutil oauth adapter for mediawiki oauth1

Summary: This is based on @csteipp's proof of concept implementation
and also borrows from the JIRA adapter in libphutil.

Test Plan: created a consumer in mediawiki and approved the
           registration. Then added the provider in phabricator
           and provided the keys from mediawiki.
           Proceeded to link my phabricator account to a
           mediawiki login.

Reviewers: qgil, epriestley, csteipp, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9202

Change-Id: I7c996145b8ce2f0453b52f9fd1fbc08e5f2b7408