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_