Fix #2302
Merge pull request #2303 from metalivedev/2302-fixintro

Fix #2302
Clean up level 3 headers.
Place 'top' subcommand's help in proper order.
Minimal, unintrusive implementation of a cleaner Job API.

* Implement a new package: engine. It exposes a useful but minimalist job API.
* Refactor main() to instanciate an Engine instead of a Server directly.
* Refactor server.go to register an engine job.

This is the smallest possible refactor which can include the new Engine design
into master. More gradual refactoring will follow.