Update: Plugin provided assemblies

Update assemblies which are available to all plugins.

Disco.Services/Plugins/PluginManifest.cs

@@ -145,8 +145,12 @@ namespace Disco.Services.Plugins
                 "Disco.Web",
                 "Disco.Web.Extensions",
                 "EntityFramework",
+                "Exceptionless",
+                "Exceptionless.Models",
+                "Exceptionless.Mvc",
                 "itextsharp",
                 "LumenWorks.Framework.IO",
+                "MarkdownSharp",
                 "Microsoft.AspNet.SignalR.Core",
                 "Microsoft.AspNet.SignalR.SystemWeb",
                 "Microsoft.Owin",