Emacs: Add address completion based on company-mode

Emacs: Add address completion based on company-mode

When company-mode is available (Emacs >= 24), address completion
candidates are shown in a nice popup box. This is triggered either by
pressing TAB or by waiting a while during typing an address. The
completion is based entirely on the asynchronous address harvesting
from notmuch-address.el so the GUI is theoretically not blocked for
long time.

The completion works similarly as the TAB-initiated completion from
notmuch-address.el, i.e. quick harvest based on user input is executed
first and only after full harvesting is finished, in-memory cached data
is used.

[Improved by David Bremner]