Add submodule support and fix src paths for examples

Since auth.ini files were included in the repo, changes to them would cause submodule updates to fail. By renaming them to auth-dist.ini and adding auth.ini to .gitignore users can simply add this repo as a submodule to their project as I have. Just copy auth-dist.ini to auth.ini and update it with your codes/info to get started.

"Fix src paths for examples" is a duplicate of pull request [#39](https://github.com/googleads/googleads-php-lib/pull/39) for [googleads/googleads-php-lib](https://github.com/googleads/googleads-php-lib), but I thought I would include them as a "complete solution" for myself until these changes are pulled.