Improved Paypal-IPN-Plugin

[josegonzalez]

I've merged in the following two items:

Re-applied: Allow loading existing IPN record, based on tnx_id or parent_tnx_id

• Simplified the InstantPaymentNotification::searchIPNId() method from original implementation
• Thanks to cali1/CakePHP-Paypal-IPN-Plugin@2b76b98 on github

Re-applied: Add support for Encrypted Web Payments

• Instructions at https://cms.paypal.com/cgi-bin/marketingweb?cmd=_render-content&content_ID=developer/e_howto_html_encryptedwebpayments#id08A3I0P017Q
• Allows user to override encryption array on a per-checkout basis
• Moved encryption arrays into hashes per PaypalIpnConfig
• Thanks to shihab-alain/CakePHP-Paypal-IPN-Plugin@3ef4bef on github

Also did the following awesome commit:

improved button handling
  - Added error-handling code to InstantPaymentNotification model and Helper
  - Allow loading of config from Configure::read() instead of App::import()
  - Allow overriding all configuration via an embedded array in passed options
  - Proper PHP5.3 Support
  - Allow passing options to submit button for formatting
  - Wrap App::import() calls in class_exists()

Also made some formatting changes.