Added SetCurl function to set the cURL instance externally

This pull request is for the addition of a SetCurl function, to allow the cURL instance to be set externally and thus allow any arbitrary cURL options to be used, as well as a change to the Connect function to check if $this->ch is a resource already before initializing it.

Let me know if this looks good!

For https://github.com/troydavisson/PHRETS/issues/34