Add cmf:media + cmf:image mixin

- adds unit tests for phpcr File and Image model
- fixes elFinder to add files in a subfolder

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10, #57
| License       | MIT
| Doc PR        | n/a

**TODO**
- [x] check fixed tickets
- [x] update changelog and describe migration command, the gist is not finished but here is a start: https://gist.github.com/rmsint/6776300
- [x] move hierachyInterface to phpcr-odm and update Folder::addChild method