Multiple Requests Behavior Changes

Change default request recording behavior for multiple requests. If you make the same request multiple times to the same URL, the response might be different each time (maybe the response has a timestamp in it or something), so this will make the same request multiple times and save them all. Then, when you are replaying the cassette, the responses will be played back in the same order in which they were received. If you were making multiple requests to the same URL in a cassette before version 0.4.0, you might need to regenerate your cassette files. Also, removes support for the cassette.play_count counter API, since individual requests aren't unique anymore. A cassette might contain the same request several times. Also removes secure overwrite feature since that was breaking overwriting files in Windows, and fixes a bug preventing requests's automatic decompression from working