The current master and alpha versions use std::future with async/await syntax,
and requires a beta compiler of version 1.39.0. 0.6.0 works with stable and futures 0.1.
Bugfixes for the stable release should go against the v0.6 branch.
Check out the examples directory for a simple sender.
To see it used in a real project, take a look to the XORC Notifications, which is a full-fledged consumer for sending push notifications.