Fixed #6 - !lt is not same as gt

Thanks to @qfox for finding this.

TODO: The tests aren't currently hard-testing this property, as gt() is
only used in a branch and bound problem.

fd.js

@@ -888,7 +888,7 @@ var FD = (function (exports, Math) {
 
     // Greater than propagator.
     Space.prototype.gt = function (v1name, v2name) {
-        return this.lt(v2name, v1name);
+        return this.lte(v2name, v1name);
     };
 
     // Less than or equal to propagator.
@@ -943,7 +943,7 @@ var FD = (function (exports, Math) {
 
     // Greater than or equal to.
     Space.prototype.gte = function (v1name, v2name) {
-        return this.lte(v2name, v1name);
+        return this.lt(v2name, v1name);
     };
 
     // Ensures that the two variables take on different values.