The stock market keeps hitting new heights and 288 new billionaires were minted last year, those with cash are eager to flash it. For the nouveau riche, the best way to make a statement about their wealth is still through art, leading to a $2.5 billion bumper sales season at the major New York auction houses — more than double last year’s tally when Christie’s, Sotheby’s and Phillips sold $1 billion during the same period. Billionaires have been chasing trophy works by 20 th century