Continuous integration

[Dominic Hamon]

I've set up a continuous integration instance at drone.io for dartbox2d. You can follow along at https://drone.io/dominic/dartbox2d

Every commit should start a build.

It doesn't use the ant pipeline, sadly, due to missing dependencies, but it will at least run the analyzer and JS generation on a subset of the examples. I may add more as time allows.