vendor: add sourcegraph/s3cache and dependencies

Adds:
 - github.com/kr/http/transport
 - github.com/sqs/s3
 - github.com/sqs/s3/s3util
 - sourcegraph.com/sourcegraph/s3cache

vendor/github.com/sqs/s3/client.go

@@ -0,0 +1,28 @@
+package s3
+
+import (
+	"net/http"
+	"time"
+
+	"github.com/kr/http/transport"
+)
+
+// Client returns an HTTP client that signs all outgoing requests.
+// The returned Transport also provides CancelRequest.
+// Client is equivalent to DefaultService.Client.
+func Client(k Keys) *http.Client {
+	return DefaultService.Client(k)
+}
+
+// Client returns an HTTP client that signs all outgoing requests.
+// The returned Transport also provides CancelRequest.
+func (s *Service) Client(k Keys) *http.Client {
+	tr := &transport.Wrapper{Modify: func(r *http.Request) error {
+		if r.Header.Get("Date") == "" {
+			r.Header.Set("Date", time.Now().UTC().Format(http.TimeFormat))
+		}
+		s.Sign(r, k)
+		return nil
+	}}
+	return &http.Client{Transport: tr}
+}