Remove secret_key config, not needed with the cookie session

Docker-DCO-1.1-Signed-off-by: Sam Alba <sam.alba@gmail.com> (github: samalba)
Remove session creation and unused session checks

Docker-DCO-1.1-Signed-off-by: Sam Alba <sam.alba@gmail.com> (github: samalba)
Adapt unit tests to reflect recent changes on Payload-Checksum

Docker-DCO-1.1-Signed-off-by: Sam Alba <sam.alba@gmail.com> (github: samalba)
Remove cookie session code (closes #339 #330)

Docker-DCO-1.1-Signed-off-by: Sam Alba <sam.alba@gmail.com> (github: samalba)
Cleanup some unused deprecated to secret_key

Docker-DCO-1.1-Signed-off-by: Sam Alba <sam.alba@gmail.com> (github: samalba)
Cleanup old references to secret_key

Docker-DCO-1.1-Signed-off-by: Sam Alba <sam.alba@gmail.com> (github: samalba)
Merge pull request #351 from dotcloud/remove-cookie

Remove cookie (flask.session)