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