fix sorting with null optional value

Add Spanish translation

I didn't think to check the tests . . . sorry!

Return a 403 if a view class's is_accessible() returns false.

is_accessible says: Override this method to add permission checks.
This means that it should return forbidden since the item exists but you
are not allowed to see it

Fixed #318.

Datetimepicker

Add extra documentation for `column_formatters`

refs #3, fixed sqlalchemy.exc.DataError when saving empty AjaxSelectMultipleField

The mongoengine auth example won't work TypeError and AttributeError

Fixed error : raise TypeError(repr(o) + " is not JSON serializable") , TypeError: ObjectId('') is not JSON serializable

Fix Select2TagsField and Remove ModelView.column_hide_backrefs