Update Published S3 Asset Structure

In https://github.com/stefanpenner/ember-app-kit/pull/60#issuecomment-22517501 @stefanpenner suggested a new layout for files published to S3:

```
builds.emberjs.com/<ref>/<files>

# for example
builds.emberjs.com/<sha>/<files>
builds.emberjs.com/v1.0.0/<files>
builds.emberjs.com/latest/<files>

builds.emberjs.com/stable/ember.js
builds.emberjs.com/stable/ember.prod.js
builds.emberjs.com/stable/ember.min.js
```

This PR is a first cut to handle the `latest` and `<sha>` pieces.  

I am continuing to work on adding the git `tag` and `stable` support functionality, but I wanted to get the PR started so that I can get some feedback.