implement "raw" requests

This attempts to provide a solution for sending "raw" requests via the cURL class, for use in APIs (such as the eBay trading API) which do not expect POST data to be in the form of URL parameters, and instead accept raw XML.

It is worth noting that these changes are technically incompatible with version 0.5.0: Though older code which passes a boolean to the ->setJson() and ->newRequest() methods will continue to work, any code which operated on truthy-numbers (eg: 1 vs 3) may not work with this version of the code.

in summary:
 $r->setJson(TRUE);  // still works
 $r->setJson(17); // previously set Json to 'true', now throws an InvalidArgument exception