redirect_uri is optional for WebServer Client

In specifications of OAuth2.0(RFC6749), access token request requires redirect_uri param.
However, Google supports a request which doesn't include it.
https://developers.google.com/accounts/docs/CrossClientAuth?hl=en
"Android app obtains offline access for web back-end"
Merge pull request #19 from ritou/redirect-uri-is-optional

redirect_uri is optional for WebServer Client