add .hash() function for a block

This is the plain old hash, which is a double sha256. The id of a block is the
reverse of this.

lib/block.js

@@ -94,8 +94,12 @@ Block.prototype.toBufferWriter = function(bw) {
   return bw;
 };
 
+Block.prototype.hash = function() {
+  return Hash.sha256sha256(this.blockheader.toBuffer());
+};
+
 Block.prototype.id = function() {
-  return BufferReader(Hash.sha256sha256(this.blockheader.toBuffer())).reverse().read();
+  return BufferReader(this.hash()).reverse().read();
 };
 
 module.exports = Block;

test/block.js

@@ -127,6 +127,16 @@ describe('Block', function() {
 
   });
 
+  describe('#hash', function() {
+    
+    it('should return the correct hash of the genesis block', function() {
+      var block = Block().fromBuffer(genesisbuf);
+      var blockhash = new Buffer(Array.apply([], new Buffer(genesisidhex, 'hex')).reverse());
+      block.hash().toString('hex').should.equal(blockhash.toString('hex'));
+    });
+
+  });
+
   describe('#id', function() {
     
     it('should return the correct id of the genesis block', function() {