bigtable: add license to file

Change-Id: I66cdb1d8081cbdac473b2847344f76491c94b0cf
Reviewed-on: https://code-review.googlesource.com/9724
Reviewed-by: Gary Elliott <garyelliott@google.com>

bigtable/export_test.go

@@ -1,15 +1,32 @@
+/*
+Copyright 2016 Google Inc. All Rights Reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
 package bigtable
 
 import (
-	"cloud.google.com/go/bigtable/bttest"
 	"errors"
 	"flag"
 	"fmt"
+	"strings"
+	"time"
+
+	"cloud.google.com/go/bigtable/bttest"
 	"golang.org/x/net/context"
 	"google.golang.org/api/option"
 	"google.golang.org/grpc"
-	"strings"
-	"time"
 )
 
 var legacyUseProd string