add GDrive provider support (#118)

* GDrive provider support
* More reliable basedir ownership
* Fix mimetype

vendor/google.golang.org/api/internal/creds.go

@@ -0,0 +1,42 @@
+// Copyright 2017 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 internal
+
+import (
+	"fmt"
+	"io/ioutil"
+
+	"golang.org/x/net/context"
+	"golang.org/x/oauth2/google"
+)
+
+// Creds returns credential information obtained from DialSettings, or if none, then
+// it returns default credential information.
+func Creds(ctx context.Context, ds *DialSettings) (*google.DefaultCredentials, error) {
+	if ds.Credentials != nil {
+		return ds.Credentials, nil
+	}
+	if ds.CredentialsFile != "" {
+		data, err := ioutil.ReadFile(ds.CredentialsFile)
+		if err != nil {
+			return nil, fmt.Errorf("cannot read credentials file: %v", err)
+		}
+		return google.CredentialsFromJSON(ctx, data, ds.Scopes...)
+	}
+	if ds.TokenSource != nil {
+		return &google.DefaultCredentials{TokenSource: ds.TokenSource}, nil
+	}
+	return google.FindDefaultCredentials(ctx, ds.Scopes...)
+}