Add pending tasks to cluster health and cat health

Closes #1364

Author: gmarz

src/Nest/Domain/Cat/CatHealthRecord.cs

@@ -37,5 +37,8 @@ public class CatHealthRecord : ICatRecord
 
 		[JsonProperty("unassign")]
 		public string Unassigned { get; set; }
+
+		[JsonProperty("pending_tasks")]
+		public string PendingTasks { get; set; }
 	}
 }

src/Nest/Domain/Responses/HealthResponse.cs

@@ -17,6 +17,7 @@ public interface IHealthResponse : IResponse
 		int RelocatingShards { get; }
 		int InitializingShards { get; }
 		int UnassignedShards { get; }
+		int NumberOfPendingTasks { get; }
 		Dictionary<string, IndexHealthStats> Indices { get; }
 	}
 
@@ -50,7 +51,8 @@ public HealthResponse()
 		public int InitializingShards { get; internal set; }
 		[JsonProperty(PropertyName = "unassigned_shards")]
 		public int UnassignedShards { get; internal set; }
-
+		[JsonProperty(PropertyName="number_of_pending_tasks")]
+		public int NumberOfPendingTasks { get; internal set; }
 		[JsonProperty(PropertyName = "indices")]
 		[JsonConverter(typeof(DictionaryKeysAreNotPropertyNamesJsonConverter))]
 		public Dictionary<string, IndexHealthStats> Indices { get; set; }