V1.3.8: Add debug messages to the update system (disabled by default); Remove debug code that causes problems in strict mode

V1.3.7debug1: Debug version for Blaine Rittmer: Downloading the update information works, converting to JSON seems to fail, so we need to add debug statements in that area...

  * [Shipping] (shipping costs)
  * [ShippingMethods] (user-readable string)
  * [ShippingMethodIDs] (internal, unique IDs for shiping methods, like shipping_by_rules:31)
  * [ShippingMethodTypes] (shipping plugin names)
  * [ShippingInstanceIDs] (unique identifiers for shipping instance when using zones)

Fixes #13

Fixes #14.

V1.3.6: Fix order numbers for old orders; Fix JS for update credentials to include the slug and thus work when two OpenTools plugins are installed

V1.3.5: Added variables [MonthName], [MonthName3], [Week], [WeekNumberYear], [DayOfYear], [Weekday], [WeekdayName], [WeekdayName3] (advanced version only)

- Order tracking assumed the user entered the order ID => implement filter to convert the entered order number into the corresponding ID
- Added trigger woocommerce_order_id_from_number(ordernumber) that returns the order ID if found (if not found, returns ordernumber unchanged)

V1.3.1: Support for generic invoice number API (http://www.open-tools.net/blog/63-proposal-for-a-general-api-for-third-party-woocommerce-invoice-numbering-plugins.html); Implementation for five different invoicing plugins

Support for almost all third-party invoice plugins (patches ready to be submitted to the respective authors, this plugin needs hardly any change)

-) Add filters to query whether a third-party plugin claims responsibility for invoice numbers
-) Add filter to get the configuration link for this plugin
-) Move all configuration / documentation links to a class-wide property to avoide duplication
-) Generic function to check whether order/invoice numbers are enabled

Move all credential check data into html data- attributes so that the JS will also work with multiple plugins. The AJAX calls will simply retrieve the data from the button's attributes, including the ajax url and the list of possible credentials