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/s3util/open.go

@@ -0,0 +1,33 @@
+package s3util
+
+import (
+	"io"
+	"net/http"
+	"time"
+)
+
+// Open requests the S3 object at url. An HTTP status other than 200 is
+// considered an error.
+//
+// If c is nil, Open uses DefaultConfig.
+func Open(url string, c *Config) (io.ReadCloser, error) {
+	if c == nil {
+		c = DefaultConfig
+	}
+	// TODO(kr): maybe parallel range fetching
+	r, _ := http.NewRequest("GET", url, nil)
+	r.Header.Set("Date", time.Now().UTC().Format(http.TimeFormat))
+	c.Sign(r, *c.Keys)
+	client := c.Client
+	if client == nil {
+		client = http.DefaultClient
+	}
+	resp, err := client.Do(r)
+	if err != nil {
+		return nil, err
+	}
+	if resp.StatusCode != 200 && resp.StatusCode != http.StatusPartialContent {
+		return nil, newRespError(resp)
+	}
+	return resp.Body, nil
+}