Added exception handling

Added error doc mapping
Moved more logic into Session middleware
Added Credit example to app.psgi
Made StackTrace and Debug panel automatically turn on when debug mode enabled
Fixed errorHandler

lib/WebGUI.pm

@@ -220,7 +220,7 @@ sub handle {
             # non-empty output should be used as the response body
             elsif (defined $output && $output ne "") {
                 # Auto-set the headers
-                $session->http->sendHeader; # TODO: should be renamed setHeader
+                $session->http->sendHeader;
                 
                 # Use contentHandler's return value as the output
                 $session->output->print($output);