Change bidData in Auction::Respone to Json object instead of plain Strin...

Currently, exchange connector can only get the creative information from agentConfig object, which needs to be shared across different process, e.g: Router/PAL/Agent. It's not good to support application which has big number of creatives, which will make the configure object pretty huge. And, basically the idea is to give the flexibilty of agent to pass customized information to exchange connector on the fly.

Thanks Remi help me to get through the whole logic.
I looked a bit more into this and it turns out that we can't just remove this line. It's eventually used to send data to the logger. So to make this work, you'll need to create a description for the bids object. To do this, you'll need to add a description macro (https://github.com/rtbkit/rtbkit/blob/master/rtbkit/common/auction.h#L308) and a createDescription function (https://github.com/rtbkit/rtbkit/blob/master/rtbkit/common/auction.h#L196) to the Bids and Bid object. As for the implementation of the createDescription function, just use this function as an example.