Add/update/delete permission targets.

:construction: 

WIP for supporting permission targets.

- [ ] tests

cc @opscode/release-engineers 
The `PermissionTarget#abbrev` and `PermissionTarget#abbreviate` methods are _really_ close and not very descriptive. Perhaps we can rename to:

* `PermissionTarget#abbrev` -> `PermissionTarget#abbreviate_permissions`
* `PermissionTarget#abbreviate` -> `PermissionTarget#abbreviate_principals`