Add a new dependency: trollius

Trollius is a port of the new asyncio module on Python 2.6 and 2.7. I would
like to replace eventlet with asyncio (Trollius). See my blueprint for Oslo
Messaging for more information:
https://wiki.openstack.org/wiki/Oslo/blueprints/asyncio

Limit the version to < 0.2 because the API *might* be broken in version 0.2.

Change-Id: Ic5d1df60eb3b86696f4c61d609e617c2ed05518c