Allow extensions to expose payment notification method

Payment processor extensions cannot reliably provide payment notification (IPN) functionality out of the box.

0. CiviCRM requests write access to the extensions directory through a warning on the admin screen.
1. The extensions directory may be in a location which is inaccessible to the web, or
2. The extensions directory may not be executable if placed within Drupal's sites/example.org/files folder (SA-2006-006)
3. The relative path to civicrm.config.php is not known to the author of the extension, but most paymentNotify.php implementations start by requiring that file.

According to a comment on the forum, one of the goals of the extensions framework was to prevent CiviCRM upgrades from breaking payment notify functionality (eg missing custom notify.php handlers in the extern directory).

I suggest we implement a custom path in CiviCRM which processors requiring IPN may use. For example, the JohnnyCash payment processor might have civicrm/paymentNotify/johnnyCash as its payment notify callback, which would in turn execute CRM_Core_Payment_johnnyCash::paymentNotify().

This was discussed in http://forum.civicrm.org/index.php/topic,21543.0.html