strategy: add strategy super class.

Gneral strategy super class which defines the most common methods
used to interact with main framework. This is the topest class for
any futures or stocks strategy.

Signed-off-by: Zhengwang Ruan <ruan.zhengwang@gmail.com>
strategy: add futures super class

Futures strategy super class which defines the most common methods
used to do futures business. Any futures strategy must inherit this
class so that the main framework could know how to interact with
a certain strategy.

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