Ungzip recorded responses if --ungzip-recorded-responses cli param given

Hi,

This PR (hopefully) closes issue #56: it adds a new command line arg, `--ungzip-recorded-responses`, which decompresses gzipped recorded responses (and removes the "Content-Encoding: gzip" header from the stored mapping).

One thought I've had is that this is maybe a bit gzip-specific. Maybe it should just be `--decompress-recorded-responses` or something, in case WireMock ever ends up supporting any other kinds of decompression, so the cli args don't need to change? Or maybe I'm over-thinking that.

As always, happy to implement any changes you might suggest, if I can.

Thanks,
Rowan