relax nokogiri dependency

This is required to make the move to XMPP a little smoother.
Blather (used by the backend's xmpp branch) depends on Nokogiri < 1.6 .
Finish all prepared statements.

A couple minor refactors as well.  Fixed a logic error (memoizing a method that takes options).
If present, pass $m4dbi_cache_id to M4DBI::Model.cached_fetch .
Use "fix leak" branches of rdbi, rdbi-driver-postgresql and m4dbi.
Bump gems: rdbi-driver-postgresql, m4dbi .
No, really bump rdbi-driver-postgresql .
Adjust dependencies in gemspec, so Bundler installs the right versions of things in libertree-backend-rb and libertree-frontend-ramaze .
Merge remote-tracking branch 'origin/master' into xmpp

Conflicts:
	libertree-model.gemspec