include docker image generation in travis builds, push to dockerhub

[rochaporto]

It seemed trivial but it's not easy to run docker from inside travis.

This is though the best way to get the build deliverables pushed to dockerhub, as we can make use of the generated binary directly. The alternative of using dockerhub automated build has the drawback of a rebuild, and the generation of much larger container images, as all the golang build tools get included and are not necessary at runtime.

See #55 for additional details on what was tried.