add attr() interface to layer

[jacquesqiao]

fix: #1811

import paddle.v2 as paddle


x = paddle.layer.data(name='x', type=paddle.data_type.dense_vector(13))
y_predict = paddle.layer.fc(input=x,
                            size=1,
                            act=paddle.activation.Linear())
y = paddle.layer.data(name='y', type=paddle.data_type.dense_vector(1))
cost = paddle.layer.mse_cost(input=y_predict, label=y)

print cost.attr()
#print cost.attr().size

will get

name: "__mse_cost_0__"
type: "square_error"
size: 1
active_type: ""
inputs {
  input_layer_name: "__fc_layer_0__"
}
inputs {
  input_layer_name: "y"
}
coeff: 1.0