Export copy function

Per https://github.com/mapbox/tilelive.js/issues/95

cc @rclark 
@GretaCB what do you think about moving the `tilelive.load` logic into here as well? These are steps that all copy operations will need to take. 

Currently you need to pass src/dst URIs but it doesn't look like you're using them in this function? How about:
```javascript
tilelive.copy = function(src, dst, opts, callback) { ... }
```