Add tests

[jasonwhite]

Basic functionality of Button needs automated test cases. I need to figure out a clean way of doing this, preferably without adding any new dependencies.

Implicit dependency detection also needs automated tests. This can be very structured -- after running the command X, test if the set of inputs/outputs contains Y files and not Z files. Adding another sub-command to run a particular command, printing inputs/outputs, might make this much easier.

[mihails-strasuns]

One idea for testing is to run tests inside docker containers and make use of docker diff to check all changes to filesystem on a given command.