refactor project

With this changes, we can now build project with pure-go.

Here is an example: http://gobuild.io/download/github.com/guilhem/helixdns

We can now build helixdns directly with go command (or gopm to manage future dep).

The only problem: resulting command is named `helixdns`. But `go build` can be modified to support renaming file output.

I'm not a go expert, don't hesitate to criticize :)