Allow task loading from Rakefile for gems (plugin installation already auto-detects). Add skip_on_db_migrate option as well for people that don't want it.

This is an interesting feature but it needs an option to not run in the
production environment.  Also, I think that the annotation task should be
manually initiated to minimize noise in commits that contain functional
changes.

Because my editor is set to do whitespace cleanup on write

option to control if should run annotate on db:migrate. Only tested with annotate_models as a plugin, not as a gem.

To control this option in your rails environment declare a module ANNOTATE_MODELS_PREFS with the constant UPDATE_ON_MIGRATE = false