Implement gzip/deflate response handling by saving gzipped data as base64

Fix https://github.com/assaf/node-replay/issues/21

All is in the title really. Based on `content-encoding` header, I save the binary content as `base64` in the fixture and recode it to binary on replay.
Existing tests pass and I added 2 tests to cover recording and replay.

Let me know what you think. 

Cheers
Jerome