Fix appending to an existing rev-manifest.json

- Use a regex to compare the file path to the name to determine if the
  file is the existing manifest.
- `file.path` is the full, absolute path, which did not match a source
  given in gulp.
- Update `readme.md` to use `event-stream`.

- Fixes #55
Rule of thumb: never put user input into a regexp and expect it to work ;)

We need better "is manifest" detection than "it ends with the same thing". I have a project with several manifests for different things, and only one of them is the rev manifest. Using this would break my project. I'm sure I'm not the only one :)