Add integration tests

This is relatively simplistic at the moment, but it sets up the framework for making more tests.

To run, simply install the latest version of docker (`curl -sL https://get.docker.io/ | sh`), and then execute `run-docker.sh` from the tests directory. This will build a docker image, compile the current source tree inside of it, and then run the tests. Note, the first run will be much slower, since it will be fetching/installing all of sandstorm's dependencies.

The docker build/run is a bit on the slow side at the moment because it's a completely clean build of sandstorm every time (it takes about 4 min on my dev machine). The best way to speed it up may be to have another Dockerfile that doesn't build sandstorm at all, and instead takes a pre-built sandstorm bundle as input.