feat(identifiable): make id member virtual

see #40 for discussion

Author: jaredcnance

README.md

@@ -86,6 +86,17 @@ public class Person : Identifiable
 { }
 ```
 
+If you need to hang annotations or attributes on the `Id` property, you can override the virtual member:
+
+```csharp
+public class Person : Identifiable
+{ 
+    [Key]
+    [Column("person_id")]
+    public override int Id { get; set; }
+}
+```
+
 #### Specifying Public Attributes
 
 If you want an attribute on your model to be publicly available, 

src/JsonApiDotNetCore/Models/Identifiable.cs

@@ -5,7 +5,7 @@ public class Identifiable : Identifiable<int>
 
     public class Identifiable<T> : IIdentifiable<T>, IIdentifiable
     {
-        public T Id { get; set; }
+        public virtual T Id { get; set; }
 
         object IIdentifiable.Id
         {