Add CircleCI build

[FedericoCeratto]

Add a .gox example. Run build on CircleCI and basic tests. The render of the example produces an unexpected artifact: https://14-104131256-gh.circle-artifacts.com/0/tmp/artifacts/example1.png

[guillaumechereau]

I would rather use a single Continuous Integration service. Currently goxel uses Travis, what would be the advantages of using CircleCI ?

I like the idea of having some test images and making sure the render stays the same. It will be specially important since I might update the file format in the next release, and I want to make sure we can still open old files.

[FedericoCeratto]

To me the main benefit of CircleCI is that it can publish the build artifacts. The artifact URL for each build is not predictable but it's possible to get it from the API. In this case it can be useful to inspect the generated images. Related to #67