Expose the native 'stream' module if you are on 0.10+.

The most common pattern when dealing with supporting with 0.8 and 0.10 is
```
var Readable     = require('stream').Readable || require('readable-stream').Readable
```
Directly from levelup: https://github.com/rvagg/node-levelup/blob/master/lib/read-stream.js#L6

We are all wiriting these lines over and over again. Why just not returning the native module?