add bn powermod and mulmod

test/bn_test.js

@@ -0,0 +1,22 @@
+new sjcl.test.TestCase("Bignum modular exponentiation test", function (cb) {
+  if (!sjcl.bn) {
+    this.unimplemented();
+    cb && cb();
+    return;
+  }
+  
+  var i, tv, g, x, N, v;
+  for (i=0; i < sjcl.test.vector.bn_powermod.length; i++) {
+    tv = sjcl.test.vector.bn_powermod[i];
+    try {
+      g = new sjcl.bn(tv.g);
+      x = new sjcl.bn(tv.x);
+      N = new sjcl.bn(tv.N);
+      v = g.powermod(x, N);
+      this.require(v.equals(new sjcl.bn(tv.v)));
+    } catch(e) {
+      this.fail(e);
+    }
+  }
+  cb && cb();
+});