[WIP] update fallback locales based on request

fix #44

what would be the right way for the configuration? currently we have

    doctrine_phpcr:
      odm:
        locales:
            en:
                - de
                - fr
            de:
                - en
                - fr
            fr:
                - en
                - de

for the fallback order per locale. we need two new things: 

* global list of allowed locales 
* the desired fallback mode

the clean thing would be to make the locales section contain the keys `allowed`, `default_fallback` and `fallback_mode`.