Refactor assignment logic into class

We're also decoupling the assigning and sending of issues. We're accomplishing this by adding a "delivered" flag on an assigned issue.
more efficient SQL query
cleanup
Throttle emails

If a user hasn't clicked on an email in the last 3 days, we drop them down to twice a week. If they haven't clicked in 14 days, we go to once a week.
fix tests
Merge pull request #263 from codetriage/schneems/dynamic-send

Refactor assignment logic into class