Emacs: Add address completion based on company-mode

Emacs: Add address completion based on company-mode

With this patch, address completion candidates are shown automatically
after short typing delay in a nice popup box. This requires company-mode
to be installed and it works only on Emacs >= 24. 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.

The notmuch-company.el is excluded from byte-compilation, because it
would require every person who want to compile notmuch to have
company-mode installed.