filter results of dataset eg.
    
    Query('Posts').filter(post_type='post')

def filter(self, **kwargs):
    # @TODO refactor with models as dicts
    """filter results of dataset eg.
    
    Query('Posts').filter(post_type='post')
    """
    f_field = kwargs.keys()[0]
    f_value = kwargs[f_field]
    _newset = []
    
    for m in self._dataset:
      if hasattr(m, f_field):
        if getattr(m, f_field) == f_value:
          _newset.append(m)
    self._dataset = _newset
    return self