Throw a RuntimeException if the cURL extension is not present

Drupal is using Guzzle and needs to be able to gracefully handle the case when cURL is not available. This pull request causes the HTTP client class to throw an exception in the constructor.

Drupal's issue: http://drupal.org/node/1991298

I'm not the original author of the patch; I'm just connecting the two projects. I'm happy to pass along any feedback.