Patched tests and modules to fix bugs with switch of linkback handling to linkback_trackback_autodiscovery module.

Refactored trackbacks to seperate trackback auto-discovery into a seperate module. Fixed tests and test-modules to reflect this change.  This will make it easier to support manual entries of trackbacks (in addition to autodiscovery).

Provide more meaningful feedback on trackback auto-discovery errors (when the HTML cannot be parsed by simplemxl).

Improved link-detection routine to support the URL-shortening filter (where a long link would be turned into an <a> tag with a valid href property, but an abbreviated link as the link text).

Improved tests with whole-suit test, which checks the complete function of the sending-linkback system.

Added watchdog debug entries.  Fixed PHP notices when checking for errors in drupal_http_request. Suppressed errors in loading simplexml content.

Added a trailing new-line to trackback-receipt responses, to improve readability. Fixed the Trackback-receive test (which now requires POST; GET not supported, as per Trackback standard). Fixed an assertion to use Identical instead of Equals, to validate FALSE vs NULL.

Added support for auto-discovery of Trackback support on external sites. Implemented basic trackback-sending.

Improved reception/validation of incoming Trackback requests to conform to the standard. Now requires POST (and a URL parameter), and returns a response conforming to the Trackback XML standard.

Fixed DB exceptions when sending linkbacks/checking if linkback has been sent (caused by changing DB row from 'linkback_handler' to 'handler').

Changed to reflect API changes in linkback module (e.g. use of linkback class). Fixed to support XMLRPC changes from D6 to D7. Added origin_ip field to track origin of pingbacks.