Allow hashTree to match after copyRecursivelySync.

This allows comparison of `hashTree` from a source directory against one created with `copyRecursiveSync` to match.

The primary reasons they did not match before was because we included both the initial (root) directory, and every directories `mtime`.

* The root directory was a problem since it is typically `tmp/merged-tree1223.tmp` (or something random), and including the random value
  in the keys cause it not to match.
* The directory `mtime` is an issue because we are NOT preserving the directory `mtime`'s. We are using `mkdirp` to create all of the directories
  needed, and the `mtime` for each is `Date.now()` at the time the directory was created (causing a mismatch between the original source and copied
  source).

Consider the following scenario:

```javascript
var fs      = require('fs');
var rimraf  = require('rimraf');
var helpers = require('./index');

rimraf.sync('foo');
rimraf.sync('bar');

fs.mkdirSync('foo');
fs.writeFileSync('foo/some-file-1.js', "'blah';");

var originalHash = helpers.hashTree('foo');

setTimeout(function() {
  helpers.copyRecursivelySync('foo', 'bar');

  var copiedHash = helpers.hashTree('bar');

  console.log('originalHash: ' + originalHash);
  console.log('copiedHash:   ' + copiedHash);
}, 500);
```

Prior to this change the two hashes would not match, after this change they will.