Fixes #36 Restrict the Priority attribute to classes

Author: niemyjski

Source/Shared/Plugins/PriortyAttribute.cs

@@ -4,6 +4,7 @@ namespace Exceptionless.Plugins {
     /// <summary>
     /// Used to determine plugin priority.
     /// </summary>
+    [AttributeUsage(AttributeTargets.Class)]
     public class PriorityAttribute : Attribute {
         public PriorityAttribute(int priority) {
             Priority = priority;