Replaced mysql_ functions with mysqli_ functions

Since mysql and all mysql_ related functions are being deprecated in PHP
with the intent to being removed and as ZenCart is moving away from
mysql to mysqli being the new default, all references to
mysql_(anything) are being edited to appropriate mysqli functions
instead. A check is being done by adding a single define to
\YOUR_ADMIN\includes\extra_datafiles\super_order_defines.php by checking
the current version of ZenCart (which is defined globally). If it finds
that the current ZenCart version is 1.5.2 or lower, it will still
perform mysql and any mysql related function.
General fixes for checkbox related pages

This set of changes removes the need to refresh the page for a check all
button. Instead, it uses Javascript to check all checkboxes on a page by
adding the checkbox area to a div and using JS to cycle through and
check all the checkboxes. The also removes the $_GET['checked'] and
$checked related variables.
Replaced mysql_query lookups with

Instead of using mysql_query or mysqli_query, this is replaced with
using ZenCart's innate $db->execute. This way it survives no matter if
ZenCart switches to mysql, mysqli or whatever.
Update README.md

Wasn't sure if this was being included in the full package out but I made a change to instruct the reader to open the file in a browser after extracting the contents of the zip file to a directory.
Update init_so_config.php

Minor mispelling.
Update invoice.php

Spelling error.
Missing Language define

super_batch_pages never had a define for 'ERROR_NO_ORDERS'
Update init_so_config.php
Removed MySQLi/MySQL switch

As suggested by lhungil, I removed the check for mysqli_ or mysqli
function. The `zen_db_perform` SHOULD be return something for
`$db->Insert_ID()` to use as this code was also prevalent in
`/includes/classes/order.php`. Also zen_db_delete no longer returns the
number of deleted fields as there is no reference to this value
whatsoever.
Version Change Noted

This wasn't updated since 4.0.4. Just adjusting the number.
Updated Readme File to reflect changes

Updated readme file to reflect version changes.
Merge pull request #1 from retched/master

MySQLi ready, Checkboxes resolved.