Improve retrieval from local folders

As noted in #1024258: Test cases...

1) On most servers (depending on whether PHP IMAP was compiled with restrictBox), imap_open() can only be passed paths relative to the webserver process's home directory. This is not ideal, and breaks testing. I will implement a workaround that makes the 'Folder' option relative to the Drupal installation directory.
2) Only mbox-format files can be retrieved.
3) The mailbox creation form needs to be updated to reflect this.