buffer: return this on int/float/double writes

This makes these write methods chainable.
```js
var buf = new Buffer(4);
buf
  .writeUInt8(0xFF, 0)
  .writeUInt16BE(0xAAAA, 1)
  .writeUInt8BE(0xBB, 3);

console.log(buf); // <Buffer FF AA AA BB>
```