Add a PhutilAsanaFuture
Summary: Ref T2852. Provides future-based access to Asana's OAuth/HTTP API.
Test Plan: Logged in with Asana, effecting a "users/me" call.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T2852
Differential Revision: https://secure.phabricator.com/D6272