[#47 #36] Serialize requests to file using gob

To replace the serial plaintext representation of request and timestamp.
This fixes a bug whereby we were unable to delimit POST requests with bodies
because they didn't end in `\r\n\r\n`, frequently resulting in:
```
--- FAIL: TestSavingRequestToFileAndReplayThem (2.80 seconds)
        integration_test.go:346: Timeout error
```

Now we don't need to play guesswork with `Content-Length` headers or
introduce a custom delimiter. As a bonus, the code required is also slightly
simpler.

It is no longer possible to manipulate the `.gor` file by hand; but it does
make it trivial to write a tool that can parse and modify.

There are some `go fmt` indent changes rolled into `replay_file_parser`.