An overhaul of the user account page

Note: #27 was the original pull request but that pull request needs to be rebased upon our current master. I did that in this pull request and resolved a few code conflicts here.

This will make it easier / possible to add additional tabs to the user
account page, and retain support for any plugins using the
'wpsc_additional_user_profile_links' hook.

I'm not 100% sure of the function placement in
wpsc-user_log_functions.php (lines 691 - 706). These might be better
implemented elsewhere, but I'm not sure where.