Maildir manipuration scripts for migration and maintenance.

- mdmerge.rb: Merge different two Maildir into one Maildir.
  Please refer the below URI for more detail:
    http://d.hatena.ne.jp/ihag/20080106#1199635378

- mh2maildir.rb: Build a Maildir from a specified MH directory.
  Please refer the below URI for more detail:
    http://d.hatena.ne.jp/ihag/20080107#1199724428

- mddigest.rb: Traverse specified Maildir, and make a message-digest
  file into it.  for more information:
    http://d.hatena.ne.jp/ihag/20080106#1199635378