add support for translation( ugettext_lazy )

Created by: ruandao

mongoform not work with ugettext_lazy for models.field.verbose_name I add some code for it, this was develop on your develop branch

e.g.

from mongoengine import Document, StringField
from django.utils.translation import ugettext_lazy as _


class XXX(Document):
    about = StringField(verbose_name=_('about'))    # the translation for 'about' will not work

on forms.py

from mongoforms import MongoForm


class XXXForm(MongoForm):
    class Meta:
        document = XXX