Use JMS\Serializer instead of GetSetNormalizer

The GetSetNormalizer was limited so we switched to JMS\Serializer. This
brings a new dependency in composer.json and changes the way you have
to write your entities.

Upgrade notes:

- you have to execute `php composer.phar update` to get the new dependency
- your entities must implement VirtualIdentity\TwitterBundle\Interfaces\TwitterEntity
- the attributes must define the JMS\Serializer\Annotation\Type annotation
- the result from twitter is flattened before its deserialized
  which means you can still use camel cased property names

Warnings:

- properties are deserialized via reflection by default, so your
  setters don't work anymore. Use JMS\Serializer\Annotation\AccessType.
- all properties found are deserialized. Exclude your auto increment
  values using JMS\Serializer\Annotation\Exclude