use WP-functions in path/dir helpers to fix #124

Deprecated the `helpers_get_dir` and `helpers_get_dir` functions in favor of the WP native `plugin_dir_path` and `plugin_dir_url` functions that do all the work of figuring out the correct path and url. 

As a bonus since WP 3.9 `plugin_dir_url` also works with symlinked plugin files so this commit also fixes #124.

I realize [ACF5 is just around the corner](https://github.com/AdvancedCustomFields/acf5-beta/blob/master/acf.php#L69) but still I think this should be added to a future release of ACF4.