Use plus/minus unicode in error msg

Figured it would be easier to read the error message if the unicode
character \u00B1 was used, instead of "+-".

lib/assert/macros.js

@@ -18,7 +18,7 @@ for (var key in messages) {
 
 assert.epsilon = function (eps, actual, expected, message) {
     if (Math.abs(actual - expected) > eps) {
-        assert.fail(actual, expected, message || "expected {expected} +-"+ eps +", but was {actual}");
+        assert.fail(actual, expected, message || "expected {expected} \u00B1"+ eps +", but was {actual}");
     }
 };