Add Slack authentication adapter

Summary: This adds an authentication adapter to libphutil for Slack. Diff 1 of 2.

Test Plan: Create a new auth provider for Slack. Link Slack account to phacility.slack.com. Authenticate a new user, create an account with Slack reg provider.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D16495