Create _bootstrap_file_scan_directory() helper function to cache results

Currently bootstrap_js_alter() calls file_scan_directory() on each page load. Granted if JS/page caching is enabled, this is less of deal. However, it would be better to create a helper function named _bootstrap_file_scan_directory(). This helper function will wrap file_scan_directory() and cache the results in {cache} with the cid of theme_registry:bootstrap:files.