Gregory Selden took on AirbnbEliot Dudik/New York Times / Redux / eye vineBy Aviva RutkinGREGORY SELDEN was planning a weekend trip with friends to Philadelphia, so he decided to book through holiday rental site Airbnb. Selden, a 25-year-old black man, enquired about a room, but was told by the host that it was not available. As he browsed the site...