Zero-fill right shift issue

great job, but I have a suggestion

``` javascript
new Node((itvl.start + itvl.end) >>> 1);
```

is quite good untill 

``` javascript
(itvl.start + itvl.end) <== 4294967295
```

because 

``` javascript
4294967296 >>> 1 === 0
4294967297 >>> 1 === 0
4294967298 >>> 1 === 1
4294967299 >>> 1 === 1
4294967300 >>> 1 === 2
```

and so on 
