describe stream.Readable class

Describe node Stream.Readable class, this allows implementing of Readable streams as described in http://nodejs.org/api/stream.html#stream_class_stream_readable_1

Specifically the Counting Stream example can now be implemented in TypeScript like this:

```javascript
import stream = require('stream');
export class Counter extends stream.Readable {

    private _max = 100;
    private _index = 1;

    constructor(opts?: stream.ReadableOptions) {
        super(opts);
    }

    _read() {
        var i = this._index++;
        if (i > this._max) {
            this.push(null);
        } else {
            var str = '' + i;
            var buf = new Buffer(str, 'ascii');
            this.push(buf);
        }
    }
}

var c = new Counter();
c.on('data', function (item) {
    console.log(item);
});
```