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

@jmcastagnetto there are fatal php error on catching exception in this line: https://github.com/google/ReCAPTCHA/pull/7/files#diff-d2d9300939ae0015474c82ba71916d9cR63 - php for some strange reason don't see Exceprion class in this file.