Appnexus exchange connector

This is a first version, roughly tested with data form AppNexus (as well as with samples provided on their web page).

There might be some adjustments to make down the road -- but'd be minor since the code is in place. 

A few few more commits are to be expected, around the creative compatibly checks (need to discuss that with business). I have no ideas about performances, IMHO the current level is good enough for AppNexus Impression Bus. 

I could not get AppNexus sandbox (using their tiny URL api) to send bid requests with the element members[] set ... 
However, sending a bid response lacking a member_id(int) element causes AN to reject it.

As a notable restriction, the connector supports single tags bid requests.