Make copy() return event emitter

This way the storage logic does not need to rely on progress

src/progress.js

@@ -12,7 +12,6 @@ exports = module.exports = {
 };
 
 var assert = require('assert'),
-    config = require('./config.js'),
     debug = require('debug')('box:progress');
 
 // if progress.update or progress.backup are object, they will contain 'percent' and 'message' properties
@@ -42,8 +41,6 @@ function setDetail(tag, detail) {
     assert.strictEqual(typeof tag, 'string');
     assert.strictEqual(typeof detail, 'string');
 
-    if (config.TEST && !progress[tag]) progress[tag] = { };
-
     progress[tag].detail = detail;
 }