New: CFHTTPRequest.js withCredential support to deliver cookies in CORS request

Currently, CPURLConnection has no way to access the withCredential
property of an XMLHTTPRequest, meaning we can't utilize cross-domain
calls using CORS in our applications. By exposing the withCredential
attribute in CFHTTPRequest.js, and then in its wrapper:
CPURLConnection, I believe we will be able to do this.

So far, I have only added two methods in CFHTTPRequest.js (lines 277 -
285, getter/setter for withCredentials) to get your opinion on the idea,
and whether I should implement the next step in CPURLConnection

Refs #1296