2x faster Geometry.mergeVertices()

In my profiling I found that ```Geometry.mergeVertices()``` was a major bottleneck.  I was able to narrow down the performance issue to a single line:

```javascript
key = [ Math.round( v.x * precision ), Math.round( v.y * precision ),
  Math.round( v.z * precision ) ].join( '_' );
```

I have replaced that line with this:

```javascript
key = Math.round( v.x * precision ) + '_' + Math.round( v.y * precision ) +
   '_' + Math.round( v.z * precision );
```

According to jsperf, this new version is 4x faster than the previous version on Chrome and 25% faster on Firefox and 2x faster on IE 10:

http://jsperf.com/mergevertices-core

The new version avoids creating an array, and also uses inline string concatenation rather than ```.join()```.

This results in an overall speed up of ```Geometry.mergeVertices()```of 2x in my larger scale profiling results.

EDIT: I did use bitwise truncation but I think that @WestLangley wouldn't approve in that I limited the range of the floating point vertex positions.  As such I went back to Math.round() as it still gets most of the performance increase without sacrificing anything.