Add background_filter to significant terms aggregation

Closes #1178

Author: gmarz

Diff for: src/Nest/DSL/Aggregations/SignificantTermsAggregationDescriptor.cs

@@ -40,6 +40,9 @@ public interface ISignificantTermsAggregator : IBucketAggregator
 		[JsonProperty("gnd")]
 		GoogleNormalizedDistanceHeuristic GoogleNormalizedDistance { get; set; }
 
+		[JsonProperty("background_filter")]
+		IFilterContainer BackgroundFilter { get; set; }
+
 	}
 
 	public class SignificantTermsAggregator : BucketAggregator, ISignificantTermsAggregator
@@ -54,6 +57,7 @@ public class SignificantTermsAggregator : BucketAggregator, ISignificantTermsAgg
 		public MutualInformationHeuristic MutualInformation { get; set; }
 		public ChiSquareHeuristic ChiSquare { get; set; }
 		public GoogleNormalizedDistanceHeuristic GoogleNormalizedDistance { get; set; }
+		public IFilterContainer BackgroundFilter { get; set; }
 	}
 
 	public class SignificantTermsAggregationDescriptor<T> : BucketAggregationBaseDescriptor<SignificantTermsAggregationDescriptor<T>, T>, ISignificantTermsAggregator where T : class
@@ -80,6 +84,8 @@ public class SignificantTermsAggregationDescriptor<T> : BucketAggregationBaseDes
 
 		GoogleNormalizedDistanceHeuristic ISignificantTermsAggregator.GoogleNormalizedDistance { get; set; }
 
+		IFilterContainer ISignificantTermsAggregator.BackgroundFilter { get; set; }
+
 		public SignificantTermsAggregationDescriptor<T> Field(string field)
 		{
 			Self.Field = field;
@@ -161,5 +167,10 @@ public SignificantTermsAggregationDescriptor<T> GoogleNormalizedDistance(bool? b
 			return this;
 		}
 
+		public SignificantTermsAggregationDescriptor<T> BackgroundFilter(Func<FilterDescriptor<T>, FilterContainer> selector)
+		{
+			this.Self.BackgroundFilter = selector(new FilterDescriptor<T>());
+			return this;
+		}
 	}
 }

Diff for: src/Tests/Nest.Tests.Integration/Aggregations/BucketAggregationTests.cs

@@ -63,8 +63,27 @@ public void SignificantTerms()
 					sigTermTerms.Items.Should().NotBeEmpty();
 				}
 			}
+		}
 
+		[Test]
+		public void SignificantTermsWithBackgroundFilter()
+		{
+			var results = this.Client.Search<ElasticsearchProject>(s => s
+				.Size(0)
+				.Aggregations(a => a
+					.SignificantTerms("sig_terms", st => st
+						.Field(p => p.Content)
+						.BackgroundFilter(bf => bf
+							.Term(p => p.Name, "elasticsearch")
+						)
+					)
+				)
+			);
+			results.IsValid.Should().BeTrue();
+			var bucket = results.Aggs.SignificantTerms("sig_terms");
+			bucket.Items.Should().NotBeEmpty();
 		}
+
 		[Test]
 		public void Histogram()
 		{