buffer: add 24-bit operators for buffer module

Some of APIs of `Buffer` what I added listed below:

* `buf.readUIntLE(offset, byteLength, noAssert)`

* `buf.readUIntBE(offset, byteLength, noAssert)`

* `buf.readIntLE(offset, byteLength, noAssert)`

* `buf.readIntBE(offset, byteLength, noAssert)`

* `buf.writeUIntLE(value, offset, byteLength, noAssert)`

* `buf.writeUIntBE(value, offset, byteLength, noAssert)`

* `buf.writeIntLE(value, offset, byteLength, noAssert)`

* `buf.writeIntBE(value, offset, byteLength, noAssert)`

See: http://tools.ietf.org/html/rfc5246#section-7.4
In this spec or other protocols, `uint24` type is often used then these API would simplify related protocols implementation.


`Buffer#readUIntLE()` can be simplified to the following:
```javascript
Buffer.prototype.readUIntLE = function(offset, byteLength, noAssert) {
  offset = offset >>> 0;
  byteLength = byteLength >>> 0;
  if (!noAssert)
    checkOffset(offset, byteLength, this.length);
  var val = this[offset];
  while (--byteLength)
    val += this[offset + byteLength] * (1 << (8 * byteLength));
  return val;
};
```
Also that includes a change that requires `byteLength` to be passed. Though there's no check to make sure it is being passed.