Rethink stable targets.

Without this change a push to `stable` would overwrite `latest/ember.js`, `ember-latest.js`, and `daily/YYYYMMDD/ember.js`.

This PR adds a whole new layer of output for the `stable` branch.

Full output for the following scenarios:

Stable commit:

```ruby
file = EmberDev::Publish::Asset.new('ember.js', :stable => true)
file.targets_for('.js')

=> ["stable/ember.js",
    "stable/daily/20130826/ember.js",
    "stable/shas/3385bffee116a97c555b9672b64297fe283cfa5b/ember.js"]
```

Master Commit:

```ruby
file = EmberDev::Publish::Asset.new('ember.js')
file.targets_for('.js')

=> ["ember-latest.js",
    "latest/ember.js",
    "daily/20130826/ember.js",
    "shas/3385bffee116a97c555b9672b64297fe283cfa5b/ember.js"]
```

For tagged commits the following will also be output:

```
tags/v999/ember.js
```