Adds option to generate Sprockets manifest.json for Rails.

When specifying

```js
fingerprint: {
  generateRailsManifest: true
}
```

Fingerprint will generate a Sprockets compatible manifest.json for rails
which will allow rails to auto-resolve fingerprinted assets.

Once this is added to asset-rev, ember-cli will need to be modified as
well to take advantage of this
Merge pull request #4 from danmcclain/sprockets-manifest

Adds option to generate Sprockets manifest.json for Rails.