Notification

Not intended to be pulled into master.

This PR shows how to use Chrome Extension's base notifications module in order to add notifications for spot_cx.

Short comings: there is no way to add `onClick` actions to the notification s.t. `url` information is preserved.

In order to do the above, the notification cannot use chrome's notification module, and instead must use a webkit notification object that accepts its own unique `.html`.  Then JS can act on that unique `.html`'s DOM in order to attach/remove DOM elements that would include a spinner/<a> tags relevant to the spotify URL.

This investigation concludes that injecting the "notifications" onto the page's native DOM using a Context Script is probably preferable.  See @rgsoto's gist of evernote's injected HTML solution.