Topics/env vars

Relaxed the configuration:

 * if no users or repos are provided, the GitHub API is used to "pull" the data
 * added a _test_ task to rake
 * moved class to `lib/`
Awesome. I think it might be nice to just consolidate the related logic though, so something like:
```
if ENV["GITHUB_REPOS"]
  ... # split/sort
else
  ... # do a lookup
end
```

I just feel like the logic of that might be a bit clearer/easier to follow. What do you think?