File Fetcher doesn't obey allowed extensions

The FeedsFileFetcher class implements a method, listFiles to list all the files to fetch. However, this method does not take into consideration the allowed extensions set in the Fetcher configuration. This leads to attempts to import the wrongs files.

Attached is a patch that makes two major changes. One is to ensure that only files with the correction extension are fetched. This is done by a means of the second major change, which is to swap out normal PHP file scanning techniques with Drupal techniques instead. In this instance, this means using file_scan_directory which also reduces code. Refactor++.