Fixed bug preventing ASIAuthenticationDialog from being dismissed

In iOS 5, the ASIAuthenticationDialog could not be dismissed using either the Cancel button or automatically by successfully logging in.
The bug is due to a change in UIViewController in iOS 5.  The class & instance "dismiss" methods are using [UIViewController parentViewController] which, as of iOS 5, no longer returns the presenting view controller.  A new property called 'presentingViewController' was added in iOS 5 to make up for this.

Changed the class & instance dismiss methods to use the "presentingViewController" property if it is available (iOS 5+).  If not, it falls back to the "parentViewController" property.

@@ -216,6 +216,9 @@ static const NSUInteger kDomainSection = 1;
 
 + (void)dismiss
 {
+	if ([sharedDialog respondsToSelector:@selector(presentingViewController)])
+		[[sharedDialog presentingViewController] dismissModalViewControllerAnimated:YES];
+	else 
 		[[sharedDialog parentViewController] dismissModalViewControllerAnimated:YES];
 }
 
@@ -233,6 +236,9 @@ static const NSUInteger kDomainSection = 1;
 	if (self == sharedDialog) {
 		[[self class] dismiss];
 	} else {
+		if ([self respondsToSelector:@selector(presentingViewController)])
+			[[self presentingViewController] dismissModalViewControllerAnimated:YES];
+		else
 			[[self parentViewController] dismissModalViewControllerAnimated:YES];
 	}
 }