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.
Ups version to 0.0.7
Fixes syntax error

This will require a new release, was missing a comma
Merge pull request #6 from danmcclain/patch-2

Fixes syntax error
Updating documentation for `GenerateRailsManifest`
Merge pull request #5 from danmcclain/patch-1

Updating documentation for `GenerateRailsManifest`
Ups version to 0.0.8
Correctly generates rails 4 manifest

Heroku expects a hash in the filename