Add docblock to setup method.

Added docblock header with some comments to the top of the setup
function.
Add public_path configuration option.

The location of the public_path (document root) for uploading files has
been moved into the configuration array so that it can be injected into
the attachment class.  This further reduces coupling between the
attachment object and the Laravel framework.
Create tests for the attachment class.

Created tests for the attachment class.