Varint(BN()) convenience

...and add some tests for the various constructor conveniences

lib/varint.js

@@ -10,6 +10,9 @@ var Varint = function Varint(buf) {
   } else if (typeof buf === 'number') {
     var num = buf;
     this.fromNumber(num);
+  } else if (buf instanceof BN) {
+    var bn = buf;
+    this.fromBN(bn);
   } else if (buf) {
     var obj = buf;
     this.set(obj);

test/varint.js

@@ -14,6 +14,11 @@ describe('Varint', function() {
     varint = Varint(buf);
     should.exist(varint);
     varint.buf.toString('hex').should.equal('00');
+
+    //various ways to use the constructor
+    Varint(Varint(0).toBuffer()).toNumber().should.equal(0);
+    Varint(0).toNumber().should.equal(0);
+    Varint(BN(0)).toNumber().should.equal(0);
   });
 
   describe('#set', function() {