Added support for not overwriting certain files when updating.

<package version="1.1">
  <filegroup source="http://example.com/update/" dest=".">
    <file size="1024" version="any_string" overwrite="never">example.txt</file>
  </filegroup>
</package>

If the version of the file changes, but overwrite is disabled, the old version of the file will be stored in the update log. If the overwrite flag is later removed, the file will be updated.