Add browserify 3.x support

`util.isBuffer(new Buffer(4))` currently returns false with `core-util-is` and browserify 3.x.

In short, `instanceof Buffer` doesn’t work to check if an object is a `Buffer` in browserify. In browserify, the Buffer constructor actually returns an instance of `Uint8Array` for performance reasons.

So, can we change this to use `Buffer.isBuffer` instead? That gives browserify a chance to define it’s own Buffer.isBuffer implementation. I believe that in node `Buffer.isBuffer` just does `instanceof` anyway.