Add a mine interface in chaincode

[juniwang]

Currently when somebody makes a contribution, we actually transfer project token from project address to user's address. But that's not right since project's token might use up. We need to add to chaincode a new API so that we can generate new token for certain project and directly assign to the related contributor, while at the same time keep project's token unchanged.

See https://github.com/kaiyuanshe/kcoin/blob/master/dev_guide.md#chaincode-development for how to develop on chaincode