* Added thumb-handler.php5 file
* Moved 404.php to thumb-handler.php as wfDisplay404Error()
* Removed excess short-circuiting error cases in thumb-handler.php
* Moved cURL extension check to the top of the file
* Improved sample $thgThumbHashFragment regex value
* Cleaned up wfStreamThumbViaCurl() error handling code a bit
* Changed custom functions to work via a $thgThumbCallbacks variable
* Added 'checkCache' and 'fillCache' hooks to $thgThumbCallbacks
* Minor regex cleanup to wfExtractThumbParams()
* Re-organized a bit of the wfStreamThumbViaCurl() code
* The thumb.php parameter extraction can also be overridden by the config to handle more complex setups and things like OggHandler and PagedTiffHandler.
* A simple 404 error page is also included. It can be overridden by the config.
* Additional HTTP headers can be passed through cURL via the config.
