support reading varInt nums up to js precision

Javascript only supports 64 bit floating points, which have uint precision up
to Math.pow(2, 53). We now support reading variable sized numbers up to that
size. If the number is bigger than that, then we need to use BN.

lib/bufferreader.js

@@ -87,7 +87,12 @@ BufferReader.prototype.readVarInt = function() {
     case 0xFE:
       return this.readUInt32LE();
     case 0xFF:
-      throw new Error('number too large to retain precision - use readVarIntBN');
+      var bn = this.readUInt64LEBN();
+      var n = bn.toNumber();
+      if (n <= Math.pow(2, 53))
+        return n;
+      else
+        throw new Error('number too large to retain precision - use readVarIntBN');
     default:
       return first;
   }

test/bufferreader.js

@@ -1,6 +1,7 @@
 var BufferWriter = require('../lib/bufferwriter');
 var BufferReader = require('../lib/bufferreader');
 var should = require('chai').should();
+var BN = require('../lib/bn');
 
 describe('BufferReader', function() {
   
@@ -188,14 +189,22 @@ describe('BufferReader', function() {
       br.readVarInt().should.equal(50000);
     });
 
-    it('should throw an error on a 9 byte varint', function() {
-      var buf = Buffer.concat([new Buffer([255]), new Buffer('ffffffffffffffff', 'hex')]);
+    it('should throw an error on a 9 byte varint over the javascript uint precision limit', function() {
+      var buf = BufferWriter().writeVarIntBN(BN(Math.pow(2, 54).toString())).concat();
       var br = new BufferReader({buf: buf});
       (function() {
         br.readVarInt();
       }).should.throw('number too large to retain precision - use readVarIntBN');
     });
 
+    it('should not throw an error on a 9 byte varint not over the javascript uint precision limit', function() {
+      var buf = BufferWriter().writeVarIntBN(BN(Math.pow(2, 53).toString())).concat();
+      var br = new BufferReader({buf: buf});
+      (function() {
+        br.readVarInt();
+      }).should.not.throw('number too large to retain precision - use readVarIntBN');
+    });
+
   });
 
   describe('#readVarIntBN', function() {