Test 1.5 to 1.6 Upgrades

We need to run some detailed testing on the 1.5 to 1.6 upgrade and get documentation drafted for it on the wiki.

Based on feedback from Dave Lange - there may be a problem in upgrading Preferred Communications Options data. We should also carefully check other properties that were moved from DB enums to option_value keys (e.g. gender, activity type, accepted credit cards, indiv_prefix and suffix, etc.).

The upgrade process should also attempt to read the existing civicrm.settings.php file and copy applicable settings into the DB config (civicrm_domain.config_backend). This means the user doesn't really need to replace/update their settings file - we'll read the "legacy values" stored there the first time 1.6 is initialized.

NOTE: Re Upgrade Doc... I don't think there's a need for upgraded sites to "replace" their 1.5 settings file - as we'll probably wind up w/ an approach where we completely write that file from the UI for 1.7 (hence let's not worry about the cruft).