Switch to an improved layout

Since we're going to build a set of binaries out of this repo,
and since some of them are going to share structures and functions,
try to organize the repo in a more Go-like way, building
subpackages for the common code.

This might change slightly in the future, but for the moment it
feels good enough.
Add skeleton worker

This is mostly there for validating the new diretory
structure.
Use owmapi and owmworker for binaries

Instead of using the too generic `api` and `worker` version for
binary names, add the `owm` prefix to them so that it'll be clear
what they are.
Merge pull request #15 from piffio/issue-13-better-repo-layout

Issue 13 better repo layout