Differences from Mew 2.2 are:
* Supporting Darwin.
* The structure of folder list is changed. Type "1Z" once after
upgrading to Mew 3.1
* You can enter folder search mode by typing C-s and C-r in
minibuffer.
* Off-line version of "x", namely "lx", is introduced.
* "x" became much faster thanks to a new algorithm of mew-dir-messages().
* A new range "sync" is defined.
* IMAP and NNTP are supported.
* +mdrop is renamed to $inbox.
