darkflow增加验证集

除了替换diff.zif四个py模块外，还需修改

1）defults.py中训练和验证集文件夹路径改为绝对路径

2）darkflow/net/help.py：

def build_train_op(self):
		self.framework.loss(self.out)
		self.say('Building {} train op'.format(self.meta['model']))
		- optimizer = self._TRAINER[self.FLAGS.trainer](self.FLAGS.lr)
		+ self.learning_rate = tf.placeholder(tf.float32, shape=[])
		+ optimizer = self._TRAINER[self.FLAGS.trainer](self.learning_rate)
		gradients = optimizer.compute_gradients(self.framework.loss)
		self.train_op = optimizer.apply_gradients(gradients)