[Issue249] Support for ArrayBuffer in Filer.Buffer

1. Added proxy Buffer constructor for Filer.Buffer.
2. Added tests for new Filer.Buffer.
Rather than copying properties onto the object, do this:

```js
function FilerBuffer (subject, encoding, nonZero) {
  if (subject instanceof ArrayBuffer) {
    subject = new Uint8Array(subject);
  }

 Buffer.call(this, subject, encoding, nonZero);
}
FilerBuffer.prototype = Object.create(Buffer.prototype);
FilerBuffer.prototype.constructor = FilerBuffer;
```