Oauth plugin: Add query string method to request a Service Provider

Hi guys. This is my first guzzle contribution so be indulgent if I did something wrong :)

I am working on a project which use an api protected with Oauth. I wanted to use guzzle but the thing is that api only accept request with Oauth parameters in the query string. It wasn't possible with the actual Oauth plugin so here is the PR!

As you can see [here](http://oauth.net/core/1.0/#consumer_req_param) it's the third authorized method to request a Service Provider.

What do you think?