Allow If-Modified-Since header for REST API

Allow If-Modified-Since header to be sent for compatibility with the
Salesforce REST API.
Allow requests to cloudforce.com domains
Merge pull request #4 from cwarden/allow-if-modified-since

Allow If-Modified-Since header for REST API
Merge pull request #2 from cwarden/allow-cloudforce

Allow requests to cloudforce.com domains
Allow X-User-Agent header for forcetk compatibility