update bn.js to latest version

...and correspondingly get rid of Number->String conversion

See: https://github.com/indutny/bn.js/issues/17

lib/browser/Bignum.js

@@ -4,8 +4,6 @@ var bnjs = function bnjs_extended(n) {
   if (!(this instanceof bnjs_extended)) {
     return new bnjs(n);
   }
-  if (typeof n === 'number')
-    n = n.toString();
   arguments[0] = n;
   return _bnjs.apply(this, arguments);
 };

package.json

@@ -64,7 +64,7 @@
     "mocha": ">=1.15.1",
     "sjcl": "=1.0.1",
     "hash.js": "=0.3.1",
-    "bn.js": "=0.13.2",
+    "bn.js": "=0.13.3",
     "elliptic": "=0.15.7",
     "bindings": "=1.1.1",
     "bufferput": "git://github.com/bitpay/node-bufferput.git",

test/test.Bignum.js

@@ -17,6 +17,13 @@ describe('Bignum', function() {
     var bn = new Bignum(999970000);
     bn.toString().should.equal('999970000');
   });
+
+  it('should parse numbers below and at bn.js internal word size', function() {
+    var bn = new Bignum(Math.pow(2, 26) - 1);
+    bn.toString().should.equal((Math.pow(2, 26) - 1).toString());
+    var bn = new Bignum(Math.pow(2, 26));
+    bn.toString().should.equal((Math.pow(2, 26)).toString());
+  });
   
   describe('#add', function() {