Add notification service

[pkozlowski-opensource]

Extracted from the issue #39:

@petebacondarwin

Moreover we should have a centralised message notification service, to which we can post messages that will be displayed in a nice way to the user.
Such as "You are now logged in", or "You do not have access to that", or "Unable to save project info."

@pkozlowski-opensource

Yes, this was on my mind for quite some time now. There was an interesting post on the mailing list recently when someone build a flash service. Basically a service that is a key-value store. You can add key->values to this store and those will be available after a route change, then wiped-out on the next route change.

Not sure if I'm making myself clear but basically you would have 2 hashes:

• current (filled in on the previous route)
• next - to be exposed after route change

on each successful route change next becomes current and next gets re-initialized to an empty hash.

This automatic wiping-out mechanism assures that the messages get actually cleaned up when not needed any more.

[petebacondarwin]

Covered by #68