Fix wtforms/wtforms#120: Allow custom Labels in Field's __init__

Fixes wtforms/wtforms#120 by allowing code like the following:
```python
class i18nLabel(Label):
  def __init__(self, field_id, **kwargs):
    super(i18nLabel, self).__init__(field_id, None)
    self.translations = kwargs
  
  def __call__(self, text=None, **kwargs):
    if 'for_' in kwargs:
       kwargs['for'] = kwargs.pop('for_') 
    else:
       kwargs.setdefault('for', self.field_id)

    attributes = html_params(**kwargs)
    return ''.join(HTMLString('<label %s lang="%s">%s</label>' % (attributes, lang, text)) for lang, text in self.translations.iteritems())
  
_ = i18nLabel

class ApplicationForm(Form):
  name = StringField(_('name', de='German Label', en='English Label'))
```