Add attribute translated_fields to TranslatableModelForm

Adding translated_fields to TranslatableModelForm allows properties of the translated fields to bet set dynamically, like prepending a globe icon, etc.

Example:
```
class WineForm(TranslatableModelForm):
    class Meta:
        model = Wine

    def __init__(self, *args, **kwargs):
        super(WineForm, self).__init__(*args, **kwargs)

        # Append globe icon to labels of translatable fields
        for field in self.translated_fields:
            self.fields[field].label += ' <span class="glyphicon glyphicon-globe"></span>'
```

This might then look something like this:
![screen shot 2013-10-30 at 01 07 24](https://f.cloud.github.com/assets/320033/1433543/556cbf3e-40f7-11e3-994e-e3f3f6a66c14.png)