Tracking Demographics

CivilEngineeringCentral.com is a niche job board that my partner and I created last year. We recently had a potential advertiser ask us if we had a media kit and he wanted the demographics of the visitors that visit our site. We can give him some numbers on traffic, but not demographics, and we certainly don't have any fancy pie charts separating out demographics. Is there a service or software out there that we might look at to track this kind of information with the intent of putting together a mediakit down the road? Is it even necessary?