NEWARK, N.J. — Eight-time Grammy-winning singer Lauryn Hill pleaded guilty Friday to not paying federal taxes on more than $1.5 million earned over three years.   Appearing in U.S. District Court in Newark, Hill admitted failing to file tax returns from 2005 to 2007. She faces a maximum one-year sentence on each of the three counts. She was charged three weeks ago.   Dressed in a dark jacket, wh…