Make gaufrette adapter abstract and add a phpcr specific gaufrette adapter

Follow up of https://github.com/symfony-cmf/MediaBundle/pull/16

Directly added TODO's related to the directory interface structure that will probably change a little bit, see https://github.com/symfony-cmf/MediaBundle/issues/18

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | na
| Fixed tickets | na
| License       | MIT
| Doc PR        | na