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
Rev the package and give it a different name.
Merge pull request #1 from looker/fix-append-manifest-patch

Fix append manifest patch