Allow JIRA auth adapter to export a "JIRA Future"

Summary: Ref T3687. Cleans up the API a bit so we can make JIRA calls more easily.

Test Plan: See next revision. Logged in and out with JIRA.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3687

Differential Revision: https://secure.phabricator.com/D6877