ctp:ctpagent: add variation between PreClosePrice and LastPrice

Make easier to see the variation between PreClosePrice and LastPrice.

Signed-off-by: Zhengwang Ruan <ruan.zhengwang@gmail.com>
ctp: remove useless agent.py and instrument.py

Remove useless agent.py and instrument.py since they were only
used to port API from lovelain's project.

Signed-off-by: Zhengwang Ruan <ruan.zhengwang@gmail.com>