added support for offset when queries seasons in newznab,

NzbDrone.Common/IEnumerableExtensions.cs

@@ -0,0 +1,16 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace NzbDrone.Common
+{
+    public static class IEnumerableExtensions
+    {
+        public static IEnumerable<TSource> DistinctBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
+        {
+            var knownKeys = new HashSet<TKey>();
+
+            return source.Where(element => knownKeys.Add(keySelector(element)));
+        }
+    }
+}