Stream wrappers fix private urls #4202

purl was adding the site's purl as a prefix when the url was built.  I told it "stop that, bad purl!" by adding 'purl' => array('disabled' => TRUE) to the options array.

This had to happen in the stream wrapper's getExternalUrl() method.  Sites with pin already had a custom stream wrapper.  Sites without pin were using the custom drupal one.  I added a stream wrapper to os_files_private.
Merge pull request #4203 from openscholar/4202-priv_file_urls

Stream wrappers fix private urls #4202