Allowing REST responses to be returned as raw data

This allows you to set a property on your `SFRestRequest` object (`parseResponse`) to specify whether the REST response should be parsed as JSON data.  Defaults to `YES`, for backward compatibility.  When set to `NO`, the data returned will be a binary `NSData` object.

No public interface changes, but I did change the name of the response property in `[SFRestDelegate request:didLoadResponse:]` to be `dataResponse`, which is more appropriate than the previous `jsonResponse`.