Started on the UI project

Added ReactiveUI, Material design and MahApps
Added the welcome page

src/Artemis.UI/ViewModels/MainViewModel.cs

@@ -0,0 +1,21 @@
+using System.Diagnostics;
+using System.Threading.Tasks;
+using ReactiveUI;
+
+namespace Artemis.UI.ViewModels
+{
+    public class MainViewModel : ReactiveObject
+    {
+        public ReactiveCommand OpenUrl { get; }
+
+        public MainViewModel()
+        {
+            OpenUrl = ReactiveCommand.CreateFromTask<string>(OpenUrlAsync);
+        }
+
+        private async Task OpenUrlAsync(string url)
+        {
+            await Task.Run(() => Process.Start(url));
+        }
+    }
+}