Allow *not* to use cache to prevent "too many open files" error

QNetworkDiskCache seem to be leaking file descriptors on some systems (?). I have seen this on my servers and it was reported in Ghost.py issues for a number of times too. As a workaround I patched Ghost class to allow no caching. This definitely will have impact on repeated requests, but if you fetch a lot of different sites it will prevent the process from crashing.