Bump the base SDK and deployment target to 10.8

This required working around an incompatibility in base/mac/foundation_util.h.
Add bare-bones HTML notifications support

Calling Notification.show() now works. But Notification.close() does nothing,
and none of the notification's events fire.
Fire a `show` event when a notification is shown
Move notification-related code into a new NotificationPresenter class
Turn NotificationPresenter into an abstract base class

This will allow us to have Mac-specific member variables without a bunch of
ifdefs.
Show notifications even when the app is focused

It's a little icky to be taking over global state (NSUserNotificationCenter's
delegate) like this, but until we have evidence that it gets in someone's way
it's the pragmatic thing to do.
Add CreateUserNotification helper function
Don't fire the `show` event until the notification is actually shown
Fire a `click` event when a notification is clicked
Make NotificationID more useful
Implement Notification.close()