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