add BN.prototype.fromBuffer

lib/bn.js

@@ -31,6 +31,13 @@ bnjs.fromBuffer = function(buf, opts) {
   return bn;
 };
 
+bnjs.prototype.fromBuffer = function(buf, opts) {
+  var bn = bnjs.fromBuffer(buf, opts);
+  bn.copy(this);
+
+  return this;
+};
+
 bnjs.prototype.toBuffer = function(opts) {
   var buf;
   if (opts && opts.size) {

test/test.bn.js

@@ -83,6 +83,11 @@ describe('bn', function() {
       bn.toString().should.equal('1');
     });
 
+    it('should work as a prototype method', function() {
+      var bn = BN().fromBuffer(new Buffer('0100', 'hex'), {size: 2, endian: 'little'});
+      bn.toString().should.equal('1');
+    });
+
   });
 
   describe('#toBuffer', function() {