Option to bypass URL cache

This is an optimization for downloading files, mostly. The default URL cache memory usage will steadily build up as HTTP requests are made. For large files, it builds up pretty quickly and doesn't deflate consistently. URL caching is pretty redundant for downloding files, since the files are typically saved to disk and never re-downloaded with a new request, thereby making the URL caching pointless.

Profiling shows a significant difference in live memory usage for downloading hi-res images.

Just as a note, I was implementing this in RZWebserviceRequest, and when that was done I noticed there was already a flag for it in RZFileManager, not hooked up to anything, so I just passed the flag along to RZFileManager's requests.