Generate fakes using counterfeiter

Hey guys, I think [counterfeiter](https://github.com/maxbrunsfeld/counterfeiter) is ready to to use in our tests. I re-created three of our fakes (`FakeStep`, `FakeUploader` and `FakeLogStreamer`) using the tool. These were all pretty simple, so it's not a humongous win, but they can now be re-generated automatically whenever we change the interfaces. I added a script to do this.