Remove explicit dependency on 'ido'

Now uses 'completing-read' rather than 'ido-completing-read' when
prompting for a maildir. This leaves the way open for the use of
alternative completing-read replacements, such as 'helm'
(https://github.com/emacs-helm/helm).

If the user still wants to use 'ido-completing-read' then they can do
this through 'ido-ubiquitous-mode'
(https://github.com/technomancy/ido-ubiquitous)