(#377) Add UploadUrl property to release model

This now seems to be a requirement in Octokit, so we need to make sure
to map this property from the Octokit Release to our internal model, so
that we can map it back again, when required.

This may require later refactoring, once we figure out how to handle
multiple providers.

Author: gep13

src/GitReleaseManager.Core/Model/Release.cs

@@ -17,6 +17,8 @@ public sealed class Release
 
         public string HtmlUrl { get; set; }
 
+        public string UploadUrl { get; set; }
+
         public bool Draft { get; set; }
 
         public bool Prerelease { get; set; }