Added comment describing fake_system.
Total WIP towards adding homebrew distro.
WIP on homebrew test.
Added utils.check_output_strs.
Got the very basics of homebrew distro working!
Merge branch 'master' of https://github.com/0install/0install

* 'master' of https://github.com/0install/0install:
  Bug fix: makedirs didn't handle symlinks