Directory stream wrapper

Provides a stream wrapper and media browser plugin to allow the selection of files from an arbitrary local directory defined in settings.php. For the moment at least, the directory needs to reside within the docroot of the Drupal install.

This could be useful in the following cases:

1. You have a set of 'legacy' files that you want to leave in a separate directory permanently, but also be able to reference from Drupal.

2. You have very large media files that you want to host locally but which can't be uploaded.

3. You have files that you'd like to share between different Drupal installs for example between different sites on a multi-site install.

The stream wrapper is read-only.

This is partially based on https://drupal.org/project/remote_stream_wrapper