HTML 5 notifications should displace notifications in the Unity Web API

Since Firefox now is implementing[1] HTML 5 notifications [2], implementing an Ubuntu-specific notifications API seems to be both redundant and against the spirit of evolving the open web on open standards. Therefore, it might be a good idea to deprecate the Unity Web Notifications API and replace it with plain HTML 5 notifications.

[1] https://hacks.mozilla.org/2013/04/hidpi-support-html5-notifications-parallel-js-asm-js-and-more-firefox-development-highlights/
[2] http://www.w3.org/TR/notifications/