Allow for inspection of environment and config_path.

As part of some tests I have, I need to be able to temporarily swap out the environment and config_path, eg.

```
def modify_app_constants(&block)
  old_config_path = AppConstants.config_path
  old_environment = AppConstants.environment
  yield if block_given?
  AppConstants.config_path = old_config_path
  AppConstants.environment = old_environment
  AppConstants.load!
end
```

There's no way to do this at present; this pull requests adds a couple of attribute readers (with tests) for this purpose.