Documented a regexp in Session/Url.pm

Add passthruUrls to the list of URLs that are changed by fixUrl.
Wrote a bunch of POD for fixUrl
Changed the regular expressions related to badUrl so they don't need to copy the data.  This
will speed them up.
Fixed a bug where trailing slashes would defeat the badUrl check.
Re-indented according to WGBP.
Added tests to cover all of the changes.

lib/WebGUI/Session/Url.pm

@@ -140,7 +140,7 @@ sub extras {
     my $self = shift;
     my $path = shift;
     my $url = $self->session->config->get("extrasURL").'/'.$path;
-    $url =~ s$(?<!^http:)/{2,}$/$g;
+    $url =~ s$(?<!^http:)/{2,}$/$g;  ##Remove //, unless it's part of http://
     return $url;
 }