add doctrine metadata handler for non-multilingual odm, admin extension for translatable

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #47
| License       | MIT
| Doc PR        | TODO

You can now implement TranslatableInterface on all models that should be multilang and have a sonata admin extension for it. 
If cmf_core is not configured with any locales, the TranslatableMetadataListener goes and changes all translated fields to non-translated fields. This is useful when working with phpcr-odm with no translation configured.