Added lazy JSON encoder in order to support i18n in CKEditor settings

A LazyEncoder has been implemented based on Django's `DjangoJSONEncoder`, rather than `json.JSONEncoder`, as described [here](https://docs.djangoproject.com/en/dev/topics/serialization/#serialization-formats-json).

This allows us to use `ugettext_lazy` (and other lazy functions) in `settings.py` which adds the possibility to localize strings for use in the `CKEDITOR_CONFIGS` settings etc.

**Example**

    from django.utils.translation import ugettext_lazy as _

    CKEDITOR_CONFIGS = {
      'default': {
        'stylesSet': [
          { 'name': _('Lead') , 'element': 'p', 'attributes': { 'class': 'lead' } },
          ...
        ]
        ...
      }