Add pixeldrain backend

This commit adds support for pixeldrain's experimental filesystem API.

backend/pixeldrain/pixeldrain_test.go

@@ -0,0 +1,18 @@
+// Test pixeldrain filesystem interface
+package pixeldrain_test
+
+import (
+	"testing"
+
+	"github.com/rclone/rclone/backend/pixeldrain"
+	"github.com/rclone/rclone/fstest/fstests"
+)
+
+// TestIntegration runs integration tests against the remote
+func TestIntegration(t *testing.T) {
+	fstests.Run(t, &fstests.Opt{
+		RemoteName:      "TestPixeldrain:",
+		NilObject:       (*pixeldrain.Object)(nil),
+		SkipInvalidUTF8: true, // Pixeldrain throws an error on invalid utf-8
+	})
+}