The spirit of the '60s was alive Thursday night as the Artistic Differences theater company staged a dizzying performance of the musical Hair to a sold-out crowd. Second in a series of three summer concerts entitled "Summer of Rock," Hair incorporated the entire Harlow's audience. A six-piece rock band backed a cast of 17 singers who danced through a standing-room-only crowd of more than 250 people. The "tribal rock concert" opened with a live Jimi Hendrix-like performance of the "Star Spangled Banner" reminiscent of the Woodstock version. It flowed seamlessly into "Aquarius," a hit song in 1969. Hair's plot revolves around Claude (multiple actors), an East Village New Yorker who receiv