Added WebHost, a simple Kestrel server for static pages.

[arepetti]

Implements #52

It's incredibly simple, useful to serve static pages (like Tinkwell Pulse or the future UI - still without a name). It offers a simple API call to solve the host address of the registered gRPC servers (which can then be called using JSON transcoding offered by ASP.NET).

To use it:

runner www "Tinkwell.Bootstrapper.WebHost" {
 	properties {
 		root_path: "path/to/website"
 		port_number: "4999"
 	}
}

(you can also leave the port number unspecified and one will be picked automatically but then you'll need to read the logs to find which one...)