Add back the acts_as_xapian Rake tasks

An effect of moving the acts_as_xapian plugin out of vendor/plugins is
that its Rake tasks are no longer found.  A fix for this (although
not necessarily the best) is in this commit - load everything in
lib/acts_as_xapian/tasks/*.rake in the Rakefile.
Use Rails.root in the Rakefile

It seems more robust (or certainly easier to understand on reading
the code) to make these paths relative to the Rails root rather
than File.dirname(__FILE__).
Merge remote-tracking branch 'origin/restore-xapian-rake-tasks' into rails-3-develop