put gpsbabel.rb utility function inside namespace

Although, @radeksimko, the trick itself seems to have stopped working.

@phinze, soliciting your advice on whether this is the best way to do such namespacing in principle.
> Though honestly you could probably just define it as a class method on Gpsbabel itself.

I think we can't do that, because the DSL uses `method_missing`.

Thanks for the fix; my Ruby is still very far from idiomatic — which is one of many reasons why your return would be welcome!

PS We need to change the topic on IRC.