The Los Angeles Clippers are locking up head coach Doc Rivers with a long-term contract extension.  The 52-year-old has been signed to a deal that keeps him in the fold through the 2018-19 season.  New team owner Steve Ballmer says he's excited to work with Doc for a long time as they build a championship culture.  Rivers took over as head coach in June of 2013 after an eight-year stint with the Boston Celtics.  The Clippers went 57-and-25 last season, earning the third seed in the Western Conference in Rivers' first year at the helm.  L.A. lost to Oklahoma City in six games in the conference semifinals.