Metro notifications

Introduce Metro notifications (Win 8 only).  Note that to display a metro notifications, you need to have a link installed on the start menu.  This will install an invisible link when the `debugInstall` configuration value is `true`.
Ugh, I don't know.  Right now we have two notifiers (console and metro) that can only deal with a string.  (Though the metro notifier *can* do something when clicked, which may be interesting.)  But neither can display anything more rich than that.

I realize the console one is very stupid and not particularly worth mentioning in this argument.  But if we're going to support multiple notifications, I don't want to have some complex system that can understand every type of message that every type of client could possibly send.

There's already a lot and I could easily envision some GitHub and VSO specific clients that can notify you about CI builds from recent pushes, work items created / assigned to you, test failures, etc.  I would very much hate to code this message-creation process in every notification system.

I think that the client knows what it wants to tell the end-user.

We could make Event an interface, and then let the events know how to produce the messages; or we could have Clients produce the messages...  We could have a `Content` property (or a `GetContent` function) and if we want to include links, we could do a `RichContent` that produces HTML or something...