Add SCRAM-SHA-1 support

This adds the new base class XMPPAuthenticator and the derived
classes XMPPSCRAMAuth and XMPPPLAINAuth.
They are now used for authentication from within XMPPConnection.

Also adds XMPPAuthFailedException which is thrown in appropriate places.

src/XMPPExceptions.m

@@ -211,3 +211,64 @@
 	return description;
 }
 @end
+
+@implementation XMPPAuthFailedException
+@synthesize reason;
+
++ newWithClass: (Class)class_
+    connection: (XMPPConnection*)conn
+	reason: (OFString*)reason_;
+{
+	return [[self alloc] initWithClass: class_
+				connection: conn
+				    reason: reason_];
+}
+
+- initWithClass: (Class)class_
+     connection: (XMPPConnection*)conn
+{
+	Class c = isa;
+	[self release];
+	@throw [OFNotImplementedException newWithClass: c
+					      selector: _cmd];
+}
+
+- initWithClass: (Class)class_
+     connection: (XMPPConnection*)conn
+	 reason: (OFString*)reason_
+{
+	self = [super initWithClass: class_
+			 connection: conn];
+
+	@try {
+		reason = [reason_ copy];
+	} @catch (id e) {
+		[self release];
+		@throw e;
+	}
+
+	return self;
+}
+
+- (void)dealloc
+{
+	[reason release];
+
+	[super dealloc];
+}
+
+- (OFString*)description
+{
+	OFAutoreleasePool *pool;
+
+	if (description != nil)
+		return description;
+
+	pool = [[OFAutoreleasePool alloc] init];
+	description = [[OFString alloc] initWithFormat:
+	    @"Authentication failed. Reason: %@!", reason];
+	[pool release];
+
+	return description;
+}
+@end