Port to  OAuth 2

I'd be interested in other people trying out this branch and see if it makes sense and works as expected.  I was able to do the requests I was doing before with it.

I used it like this:

    # Request method
    client = LinkedIn::Client.new(ENV["LINKEDIN_API"], ENV["LINKEDIN_SECRET"])
    session[:linkedin_state] = SecureRandom.uuid
    redirect_to client.authorize_url(redirect_uri: linkedin_auth_callback_url, state: session[:linkedin_state])

    # Callback method, should check the state set above on response
    client = LinkedIn::Client.new(ENV["LINKEDIN_API"], ENV["LINKEDIN_SECRET"])
    token = client.authorize_from_request(params[:code], redirect_uri: linkedin_auth_callback_url)