Intial drop for MoPub Exchange Connetor.
* Mostly relies on OpenRTB2.1
* Checked against MoPub's sandbox.
* TODO: check the many assumptions:
  - rtt <= 200ms
  - optional fields of BidResponse made mandatory by MoPub (e.g. iurl)
* TODO: customer integration/validation
pruned unused code
adding makefiles
Merge branch 'MOPUB' of https://github.com/jsulmont/rtbkit into jsulmont-MOPUB