Create a ClientCredentialsFlow class

_From [sh...@knewton.com](https://code.google.com/u/111335644614135614514/) on May 06, 2014 13:07:20_

I have created a class ClientCredentialsFlow, which functions like AuthorizationCodeFlow, but for client credentials. Although the steps needed with authorization code are much more complicated, it is still useful to have a Flow class for client credentials authentication:
- Support for credentialStore and/or credentialDataStore
- A single authorize() call which checks the store, gets the token request, and generates a Credential
- Setting refresh listeners (although most client credentials implementations don't support a refresh token)

I copied much of the code from AuthorizationCodeFlow, as well as a few bits from AuthorizationCodeInstalledApp.authorize()

I have a clone shawn-google-oauth-java-client which contains the code. The commit is at https://code.google.com/r/shawn-google-oauth-java-client/source/detail?r=638eeedd237134ab1665c54992bbbbfb938791fc

_Original issue: http://code.google.com/p/google-oauth-java-client/issues/detail?id=93_

Beginnen Sie mit dem Vergleich der vorhandenen Implementierungen von AuthorizationCodeFlow und AuthorizationCodeInstalledApp.authorize() mit dem Code in dem referenzierten Clone und Commit von ClientCredentialsFlow. Überprüfen Sie, wie Credential Stores, der einzelne authorize()-Aufruf und Refresh-Listener integriert werden sollten, und bestätigen Sie anschließend, dass der resultierende Ablauf die im Issue aufgeführten Verhaltensweisen unterstützt.

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