Refactor Updgrade code to provide a single mechanism to adding post-upgrade messages and alerts for the user

Currently there are a number of methods for appending post-upgrade messages that need to be shown to the users. We need to have one approach that can called from various places during the upgrade based on dynamic conditions as well as a bucket to put messages that are relevant to everyone.

Two examples of static messages for everyone for the 4.1 uprade are:
"The CiviCRM Administration menu structure has been re-organized during this upgrade to make it easier to find things and reduce the number of keystrokes. If you have customized this portion of the navigation menu - you should take a few minutes to review the changes. You may need to reimplement or move your customizations."

"Yahoo recently discontinued their geocoding and mapping API service. If you previously used Yahoo, you will need to select and configure an alternate service in order to continue using geocoding/mapping tools."

Anyone converting from 3.4.x or 4.0.x should be shown these mesages.