proposed change to copyTree() to allow pre-existing target directories

[ghost]

The CommonJS/Fileystem/A spec is vague about various behavior (unless there's standard expected behavior I'm not aware of), but having copyTree() fail because makeDirectory() fails due to an existing target directory, seems unfortunate and inconsistent with the behavior at the file level, which just overwrites existing target files (as long as they're writable).

I propose changing copyTree() to allow for preexisting directories. A less desirable (IMO) alternative would be to leave copyTree() as is and add this as, say, "mergeTree()".

  exports.copyTree = function(source, target) {
    var self = this;
    return Q.when(self.stat(source), function(stat) {
      if (stat.isFile()) {
        return self.copy(source, target);
      } else if (stat.isDirectory()) {
        return self.exists(target).then(function(dstExists) {
          var copySubTree = Q.when(self.list(source), function(list) {
            return Q.all(list.map(function(child) {
              return self.copyTree(self.join(source, child), self.join(target, child));
            }));
          });
          if (dstExists) {
            return copySubTree;
          } else {
            return Q.when(self.makeDirectory(target), function() {
              return copySubTree;
            });
          }
        });
      } else if (stat.isSymbolicLink()) {
        return self.symbolicCopy(source, target);
      }
    });
  }; 

[ghost]

Clearly I failed to link this properly to the pull request. Not sure how. Closing manually as it was assigned to issue #69.