coordsFromBbox enlarge width and height twice

Nobody has claimed this yet.

The width calculated by coordsFromBbox() is base_width \* scale *scale, which should be base_width \* scale. Let check out:

```
console.log(abaculus.coordsFromBbox(0, 2, [-180, -85.0511, 180, 85.0511], 19000, 256))
console.log(abaculus.coordsFromCenter(0, 2, {x: 0, y: 0, w: 256, h: 256}, 19000, 256))
```

What I got is :

```
{ w: 1024, h: 1024, x: 256, y: 256 }
{ w: 512, h: 512, x: 256, y: 256 }
```

We have already get the right size from here: https://github.com/mapbox/abaculus/blob/master/index.js#L42-L43

But we enlarged the size again at here: https://github.com/mapbox/abaculus/blob/master/index.js#L50-L51

And `x: 256, y: 256` is definitely not the center of `w: 1024, h: 1024`, which proves the results calculated by coordsFromBbox is wrong.

If this problem is confirmed. I will send a PR.

Start in index.js at lines 42-43 and 50-51, where the issue identifies the initial size calculation and the second enlargement. Reproduce the two console.log examples, then verify that coordsFromBbox no longer doubles the width and height and agrees with coordsFromCenter for the same bounds.

Written by the indexing model from the issue text.