OAuthDataStore impl using Java Preferences

[chanseokoh]

Java Preference Implementation of OAuthDataStore.

Misc:

• IntelliJ IDE changed pom.xml to allow using @Override.

• The Javadoc of OAuthData says that the Set for storing scopes is never null:

  /**
  * Authentication data, consisting of an access token, a refresh token, an access-token expiration
  * time, and a user email address, each of which may be null, and a set of authorized scopes that is
  * never null but may be empty.
  */

  In fact, OAuthData creates an empty set if a null set is given, and OAuthData.getStoredSet() never returns null.

[briandealwis]

LGTM.