Use 'stream' instead of 'readable-stream'

This decreases browser bundle size by nearly 100KB.

Before:
```
browserify -r bncode | wc -c
  246124
```
After:
```
browserify -r bncode | wc -c
  151471
```
This breaks compatibility in node 0.8, but we can bump the major version to 1.0.0 so it won't break people's code.