Add implementation for the negation and "has" selectors

The negation selector is specified here:
http://www.w3.org/TR/css3-selectors/#negation

The "has" selector is an extra convenience selector

HTMLKit/CSSPseudoFunctionSelector.h

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+//
+//  CSSPseudoFunctionSelector.h
+//  HTMLKit
+//
+//  Created by Iska on 07/10/15.
+//  Copyright © 2015 BrainCookie. All rights reserved.
+//
+
+#import "CSSSelector.h"
+
+@interface CSSPseudoFunctionSelector : CSSSelector
+
++ (nullable instancetype)notSelector:(nonnull CSSSelector *)selector;
++ (nullable instancetype)hasSelector:(nonnull CSSSelector *)selector;
+
+@end

HTMLKit/CSSPseudoFunctionSelector.m

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+//
+//  CSSPseudoFunctionSelector.m
+//  HTMLKit
+//
+//  Created by Iska on 07/10/15.
+//  Copyright © 2015 BrainCookie. All rights reserved.
+//
+
+#import "CSSPseudoFunctionSelector.h"
+#import "HTMLElement.h"
+
+#pragma mark - Declarations
+
+@interface CSSNotSelector : CSSPseudoFunctionSelector
+@end
+
+@interface CSSHasSelector : CSSPseudoFunctionSelector
+@end
+
+#pragma mark - Base Function Selector
+
+@interface CSSPseudoFunctionSelector ()
+{
+	CSSSelector *_selector;
+}
+@property (nonatomic, strong, readonly) CSSSelector *selector;
+@end
+
+@implementation CSSPseudoFunctionSelector
+@synthesize selector = _selector;
+
++ (nullable instancetype)notSelector:(nonnull CSSSelector *)selector
+{
+	return [[CSSNotSelector alloc] initWithSelector:selector];
+}
+
++ (nullable instancetype)hasSelector:(nonnull CSSSelector *)selector
+{
+	return [[CSSHasSelector alloc] initWithSelector:selector];
+}
+
+- (instancetype)initWithSelector:(CSSSelector *)selector
+{
+	self = [super init];
+	if (self) {
+		_selector = selector;
+	}
+	return self;
+}
+
+@end
+
+#pragma mark - Not Selector
+
+@implementation CSSNotSelector
+
+- (BOOL)acceptElement:(HTMLElement *)element
+{
+	return ![self.selector acceptElement:element];
+}
+
+- (NSString *)debugDescription
+{
+	return [NSString stringWithFormat:@":not(%@)", self.selector.debugDescription];
+}
+
+@end
+
+#pragma mark - Has Selector
+
+@implementation CSSHasSelector
+
+- (BOOL)acceptElement:(HTMLElement *)element
+{
+	for (HTMLNode *child in element.childNodes) {
+		if (child.nodeType == HTMLNodeElement && [self.selector acceptElement:child.asElement]) {
+			return YES;
+		}
+	}
+
+	return NO;
+}
+
+- (NSString *)debugDescription
+{
+	return [NSString stringWithFormat:@":has(%@)", self.selector.debugDescription];
+}
+
+@end