Add some notes about notifications
More on notifications
Add notifications table to retcon DB schema
Add dependency on mime-mail
Record notifications for non-empty diffs

Impacts the PostgreSQL storage backend only.
Clarify notifications spec and code

Address feedback from pingu in #37.
Prepare (but don't attempt to send) notifications
Process notifications through all rules
Send the mail, don't just print it
Delete notifications after sending them
Merge pull request #37 from anchor/wip-notification

Notify humans about conflicts
Export lenses for RetconOptions
Refactor, mostly adding lenses and helpers
Fix SQL fixturing
Code formatter
Merge branch 'wip-refactor-lenses-and-helpers' of github.com:anchor/retcon into wip-refactor-lenses-and-helpers