Added support for curl transport, better support for PHP5 constructs

I have added support for curl, so the call to the reCAPTCHA service can work even when behind a proxy, apart from supporting timeouts, etc. There is an example that shows how to make use of that functionality.

Also made some changes to better support PHP5 syntax:

- Use the `__contruct()` method for the constructor
- Define and use an exception instead of just using `die()` to signify a fatal condition
- Removed the `$error` var because your response object already contains that information
- Refactored the class names y example code to use namespaces

Fatal error: Uncaught exception 'Google\ReCaptcha\Exception' with message 'To use reCAPTCHA you must get an API key from <a href='https://www.google.com/recaptcha/admin'>https://www.google.com/recaptcha/admin</a>'