Better handling of XML errors when making cURL requests

Issue Summary

I've attached a patch that addresses two problems I encountered deploying the module on a live site:

1. If the GSA is running on a server using HTTPS, but without a valid certificate (some organizations don't want to pay for one that won't be publicly accessed), you need to add an additional cURL parameter to allow the connection to go through.

2. If the response received from the GSA isn't valid XML (either by bad formatting or it returns a non-XML response) the module generates simpleXML errors and can potentially cache a bad result, which will confuse other users who search for similar terms.