Add general notification HTTP endpoint for use by external client

Since we don't have access to fancy stuff like SignalR or WebSockets, the implementation is based on a simple and dumb long-polling strategy which is absolutely non-scalable but should fit our need of ~1.5 people using it simultaneously.

I have tried keeping the changes minimal by being mostly self-contained. I also tried to avoid MonoDevelop project file rewriting to a minimum.

As part of the commit is added Newtonsoft.Json project in a similar way than SmartIrc4Net.