e.g.:
```
docker -d -bip "10.10.0.1/16"
```
If set and valid, use provided in place of trial and error from pre-defined array in network.go.
Mutually exclusive of -b option.
* 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.