Use a weak-memory NSHashTable for referencing iterators in HTML Document

This breaks the retain cycle between the document and the attached node
iterator.

Fixes #4

Sources/HTMLDocument.m

@@ -20,8 +20,8 @@
 @interface HTMLDocument ()
 {
 	HTMLDocument *_inertTemplateDocument;
-	NSMutableArray *_nodeIterators;
 	NSMutableArray *_ranges;
+	NSHashTable *_nodeIterators;
 }
 @property (nonatomic, assign) HTMLDocumentReadyState readyState;
 @end
@@ -41,8 +41,8 @@
 	self = [super initWithName:@"#document" type:HTMLNodeDocument];
 	if (self) {
 		_readyState = HTMLDocumentLoading;
-		_nodeIterators = [NSMutableArray new];
 		_ranges = [NSMutableArray new];
+		_nodeIterators = [[NSHashTable alloc] initWithOptions:NSHashTableWeakMemory capacity:10];
 	}
 	return self;
 }