Add the query_params argument to the Base#fetch method.

This patch allows users to specify the extend and fields parameter in the options
argument of Base#find to fetch extra information such as changelog.

By using this, you can fetch the changelog of an issue as
follows:

issue = client.Issue.find('MYPROJ-10, {expand:'changelog'})
issue.changelog['histories'].each do |c|
 ...
end

I'm not sure this patch fits with the original design. Please review it.