Place 'top' subcommand's help in proper order.
Merge pull request #2307 from gurjeet/sorted_commands_in_help

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.