Upgrade process for v2.1 -> v2.2

Complete the upgrade process/wizard for v2.1 -> v2.2.

Starting with this upgrade, our upgrades should be run from within the civicrm session context AS A MENU ITEM - civicrm/upgrade?reset=1

This will allow us to do the following (which should be included in this implementation - and SOME of which has been added to the latest 2.1.4 upgrade):

• Upgrade menu item should be permissioned - requires CiviCRM Admin perm (we will probably include an explicit menu item / link somewhere in the admin page - but we don't want non-permissioned users to be able to run it by typing in the URL).
• Cleanup templates_c and reset session automatically (so user doesn't have to do this as separate step)
• Rebuild menus automatically
• Provide clear status message to user at the end of the process - errors (if any) OR success message OR database already upgraded message (see 2.1.4 code for text)

If upgrade has already run, we detect and give the "already upgraded message" rather than an error.

I also think we should prompt for a confirmation before beginning upgrade. This can be a Javascript confirm prompt.