An overhaul of the user account page

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.

Conflicts:
	wpsc-theme/wpsc-user-log.php
Additional classes

Added additional classes to the purchase history table, changed header
cells to <th> (instead of <td><strong>) and chose better function name.

Conflicts:
	wpsc-theme/functions/wpsc-user_log_functions.php
Deprecated functions
A few tweaks

Thanks to Gary for the suggestions. wpsc-user-log.php is much much
cleaner now.

Conflicts:
	wpsc-theme/wpsc-user-log.php
printf to escape the url

Escaping the URL and attributes as per Justin's suggestion. Considered
escaping the $tab_title, too, but felt it was important for plugin
authors to be able to send values like:
<span class="my_plugin">My New Plugin</span> Tab
Better mid-line indenting

http://codex.wordpress.org/WordPress_Coding_Standards#Indentation
Removed extract

http://core.trac.wordpress.org/ticket/22400

http://josephscott.org/archives/2009/02/i-dont-like-phps-extract-functio
n/
Remove incorrect _deprecated_function() calls.
Use add_query_arg() instead of url concatenation in wpsc_user_profile_links().
Avoid using unnecessary $i iterator in wpsc_user_profile_links().
Use include() to include "your account" templates rather than
include_once()
Appease @JustinSainton's nitpickiness by correcting the PHP comments
which should reflect hierarchical relationship between templates.
Whitespace fix
Refactor wpsc-account-downloads.php so that it contains less logic and
more template.

Also added HTML classes to the table rows, make sure the table has
<thead> and <tbody>.
Escape URL in wpsc-user-log.php.
esc_url() in wpsc-user-log.php

Fixes #73.