Add buffer class to node.js interface

I had initially tried that approach, but found that the type system was not behaving as I thought it would with overloaded constructors.

For example, given a declaration like this:

```javascript
declare class Buffer {
  constructor(array: Array<number>): void;
  constructor(size: number): void;
  constructor(string: string, encoding?: string): void;
}
```

The following was still passing type check:

```javascript
/* @flow */
var b = new Buffer(null)
```