Add Drone.io for continuous integration

Travis-CI is a well functioning CI system, but its virtualized operating
system, version Ubuntu 12.04, doesn't have prebuilt deb packages for Qt
5.2 or later, which is needed for functionality added to QJsonValue.
This means that a Qt 5.2 deb package for Qt 5.2 would have to be created
or the build script for Travis-CI would have to build Qt 5.2. Drone.io,
another continuous integration service, uses Docker and allows for
creating a custom Docker image. The custom image used contains Qt 5.3.1,
which fits requirements well. The Drone.io process that queries GitHub
commits for Kryvos is currently hosted on a VPS at
http://168.235.149.114.