Fixing path problems by making them optional

Apparently some commits I pull got into the pull request. Sorry about that (though Github says it will be fine).

I kept a lot of code for backward compatibility, but removing the path option altogether we'd cut quite a lot of complexity from the rebar code as it would boil down to:

```erlang
  case is_base_dir(RebarConf) of
  true ->
    code:add_paths(["ebin" | filelib:wildcard(rebar_config:get_xconf(RebarConf, base_dir, undefined) ++ "/deps/*/ebin")]);
  false ->
    code:add_paths(filelib:wildcard(rebar_utils:get_cwd() ++ "/../*/ebin"))
  end.
```

I've tested production mode with cb_admin as a dep alongside, works like a charm.