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
I believe you, but I don't completely understand. 

If you only include the intended manifest in your `gulp.src` call, which is either `some/path/rev-manifest.json` or matches an option passed in `rev.manifest({path: opt})`, any other manifest files shouldn't be affected, right? Even if you have multiple files called `manifest.json`, I imagine they are in different dirs?