Google Checkout PHP Sample Code

CHANGELOG:
==========

v1.3.1 09/13/2010 (pying)
  - Fixed require_once path issue
  - Fixed generated HTML to be w3c compliant
  - Modifed S2S post error messages to return error message instead of 
    full response body
  
v1.3 06/29/2010(pying)
  - Fixed server to server posting bug
  - Added polling api beta
  - Added subscriptions beta
  - Added notification history api beta
  - Added support for API V2.5
  - Added option to generate signed HTML API carts
  
v1.2.5a 09/27/2007 (ropu)  
  - Fixed bugs in Line-Item Shipping
  - Added Line-Item Shipping demos

v1.2.5 09/25/2007 (ropu)  
  - Added Carrier Calculation Support
  - Added Line-Item Shipping
  - Updated posting URLs
  - Added PHP Docs
  - Modified xml-processing/* class names to avoid conflics
  - Added buyNow button support CheckoutButtonNowCode()

v1.2.1 beta 07/17/2007 (ropu)
  - Added Digital Delivery support
  - Added server 2 server Checkout API Requests    
  - Improved demo/* files
  - Added digital and server 2 server examples (digitalCart.php and DigitalUsecase())
  - Minor bug fixes

v1.2 beta 06/12/2007 (ropu)
  - Added a log class with log levels (error, request, response) (GoogleLog)
  - Split log file into error and response log files
  - Added Correct HTTP status (2xx, 4xx, 5xx) return when errors occurrs with 
      the correct message
  - Added Google Analytics Tracking
  - Added 3rd party Tracking
  - Added CheckoutAcceptanceLogo() to GoogleCart class
  - Changed the XMLParser
  - Added CheckoutHTMLButtonCode() to print a HTML API Google Checkout Button
  - Added the "What is Google Checkout?" text under the GButton (configurable)
  - Splited GoogleReponse class into GoogleResponse and GoogleRequest
  - Improved the HttpAuthentication() method in the GoogleResponse class to 
      handle apache and ISS Basic authentication. Remove GetAllHeaders().
  - Escape and chunk GoogleMessages to avoid errors
  - Added proxy support for GoogleRequest class (Order Processing API)
  - Added MerchantPrivateData and MerchantPrivateItemData classes to allow
      sending XML tags inside those tags.
  - Added the AddAllowed*() and AddExcluded*() methods to GoogleShippingFilters
      class
  - Added CHANGELOG file
  - Added TODO file
  - Changed demo/* files to show the usage of v1.2
  - Added unitTest (everyone is invited to add more test to improve coverage :)
