Support client credentials grant type

[rakyll]

http://tools.ietf.org/html/draft-ietf-oauth-v2-31#section-1.3.4

Allowing TokenFetcher to retrieve a token without a refresh token if grant type is set to client_credentials would solve the issue.

[agtorre]

I wrote a patch for this about a week ago, no word back yet, but it seems to work well for me!

https://go-review.googlesource.com/#/c/2983/