Added Google Captcha v2 to play-authenticate-usage usage sample

[slimandslam]

I used the Java 6/7 Java.net lib, instead of Play's async WS library, for the captcha verification call to Google. Everything seems to work fine, but I did this all fairly quickly so someone should probably go over the code and make sure it's in good working order. I added instructions to the Getting Started.md file. Maybe this should be made into a separate usage example, e.g. play-authenticate-basicauth-withcaptcha ?