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
- Other minor changes