Well, it looks like you found a bug. This line (732):
sort = c.numberSorter ? c.numberSorter(x[col], y[col], dir, colMax[col], table) :
should actually look like this (replace x & y with a & b, respectively)
sort = c.numberSorter ? c.numberSorter(a[col], b[col], dir, colMax[col], table) :
Then once you do that you can use this numberSorting script (demo).
$("table").tablesorter({
theme : 'blue',
numberSorter: function (a, b, direction) {
if (a >= 0 && b >= 0) { return direction ? a - b : b - a; }
if (a >= 0) { return -1; }
if (b >= 0) { return 1; }
return direction ? b - a : a - b;
}
});