S3 brain resets brain auto-save interval

- Outsmarted myself using a separate timer for the S3 brain, as this
prevents it from responding to regular .save() calls
- Added environment variable HUBOT_S3_BRAIN_SAVE_INTERVAL, which
defaults to 30 minutes
- Improved docs
Merge pull request #715 from Iristyle/s3-mods

Fixes for S3 brain saving