Scaffold isolated storage to enable UriImageSources

This is a broken implementation that is only enough to enable
URI images.

Fixes #104

Ooui.Forms/Forms.cs

@@ -96,7 +96,7 @@ namespace Xamarin.Forms
 
             public IIsolatedStorageFile GetUserStoreForApplication ()
             {
-                throw new NotImplementedException ();
+                return new LocalIsolatedStorageFile ();
             }
 
             public void OpenUriAction (Uri uri)

Ooui.Forms/LocalIsolatedStorageFile.cs

@@ -0,0 +1,40 @@
+using System;
+using System.IO;
+using System.Threading.Tasks;
+using Xamarin.Forms.Internals;
+
+namespace Ooui.Forms
+{
+    public class LocalIsolatedStorageFile : IIsolatedStorageFile
+    {
+        public Task CreateDirectoryAsync (string path)
+        {
+            return Task.FromResult<object> (null);
+        }
+
+        public Task<bool> GetDirectoryExistsAsync (string path)
+        {
+            return Task.FromResult<bool> (true);
+        }
+
+        public Task<bool> GetFileExistsAsync (string path)
+        {
+            return Task.FromResult<bool> (false);
+        }
+
+        public Task<DateTimeOffset> GetLastWriteTimeAsync (string path)
+        {
+            throw new NotSupportedException ();
+        }
+
+        public Task<Stream> OpenFileAsync (string path, Xamarin.Forms.Internals.FileMode mode, Xamarin.Forms.Internals.FileAccess access)
+        {
+            throw new NotSupportedException ();
+        }
+
+        public Task<Stream> OpenFileAsync (string path, Xamarin.Forms.Internals.FileMode mode, Xamarin.Forms.Internals.FileAccess access, Xamarin.Forms.Internals.FileShare share)
+        {
+            throw new NotSupportedException ();
+        }
+    }
+}