Preserve file metadata (esp. timestamps) when copy static files to output folder.

Related to #1027, but only focus on static files (theme files, images, attachments).

The method [shutil.copy2](http://docs.python.org/2/library/shutil.html#shutil.copy2) will copy file metadata as well as file content. Method [shutil.copytree](http://docs.python.org/2/library/shutil.html#shutil.copytree) internally also calls [shutil.copy2](http://docs.python.org/2/library/shutil.html#shutil.copy2) rather than [shutil.copy](http://docs.python.org/2/library/shutil.html#shutil.copy2).