oauth2: Removing the inconsistent and duplicate features, better naming

- Removed Flow, flow is a nothing but options.
- Renamed Cacher to Storer.
- Removed the setter from the Transport. Store should do the initial set.
  Getter is not removed, because extra fields are available through
  Transport.Token.Extra(). It's not pleasant to implement a custom Storer
  implementation to read such values.

oauth2: Remove VMs from the AppEngine example title

google/appengine.go

@@ -7,6 +7,7 @@
 package google
 
 import (
+	"net/http"
 	"strings"
 	"sync"
 	"time"
@@ -44,7 +45,9 @@ func init() {
 func AppEngineContext(ctx appengine.Context) oauth2.Option {
 	return func(opts *oauth2.Options) error {
 		opts.TokenFetcherFunc = makeAppEngineTokenFetcher(ctx, opts)
-		opts.Transport = &urlfetch.Transport{Context: ctx}
+		opts.Client = &http.Client{
+			Transport: &urlfetch.Transport{Context: ctx},
+		}
 		return nil
 	}
 }