Update build version to flag a dirty working copy

When building with a dirty working copy, that should be made clear in
the build version.  This avoids building from a dirty working copy based
directly from a tag, where it's not obvious that the build contains
local modifications that have not been committed.
Fix #46: Remove notifo support (R.I.P.)

Notifo has finally died, so there's no reason to keep its code around
any longer.  It's probably also safe to remove the section from the
readme about migrating from the old notifo module that has been gone for
over a year now.