added zlib deflateStrategy option, default to Z_RLE
added zlib deflateStrategy option, default to Z_RLE

dokipen/node-pngjs,gagern/pngjs2,lukeapage/pngjs2,neophob/pngjs2,liangzeng/node-png,sapphire-al2o3/node-pngjs,gagern/pngjs2,neophob/pngjs2,lukeapage/pngjs2