Add release info and rollback commands

```
➜ hk help release-info
Usage: hk release-info

release-info shows detailed information about a release.

➜ hk -a hkdist release-info v78
Version:  v78
By:       me@heroku.com
Change:   Removed FAKE config vars.
When:     2013-11-26T22:14:36Z
Id:       8d68c2b3-6178-4f5c-96f0-1234567890
Slug:     e5b1ca08-9625-43b8-8d89-1234567890
```

```
➜ hk help rollback    
Usage: hk rollback <version>

➜ hk -a hkdist rollback v78    
Rolled back to v78 as v85.
```