[WIP] Permissions script handler

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| License       | MIT

* This PR fixes bug `Undefined property: Composer\Script\CommandEvent::$getIO` in `clearCache()` and `installAssets()` handlers.
* I am trying to automate setting up permissions, documented in http://symfony.com/doc/current/book/installation.html#configuration-and-setup. Currently supports only `setfacl`, but we can add `chmod` if it have sense.

Feedback is welcome.