Merge pull request #51 from alphagov/document_basic_auth_injection

[#31] Document BasicAuth injection
Merge pull request #48 from alphagov/binary_file_format

[#47 #36] Serialize requests to file using gob
[WIP] Only replay GET requests

Useful when replaying production traffic against a staging environment and
POST/PUT/etc requests may have undesired effects (notifications, duplicate
data, etc).

Implemented on replay, rather than listen, because we don't normally parse
requests in listen mode. Doing so would slow down the capture and might(?)
drop packets.