RTBKitExchangeConnector now populates the externalId in the BidResponse
per Bid
HttpBidderInterface now sends only one HTTP request and uses the Bid
extension field to lookup the correct agent and AgentConfig

Conflicts:

	rtbkit/plugins/bidder_interface/http_bidder_interface.cc
externalId is now populated as an unsigned int rather than a string in
RTBKitExchangeConnector
bidder_test now setups the correct stacks with correct exchange connectors
Merge pull request #55 from datacratic/http_bidder_interface_tweaks

Http Bidder Interface