Issue #113: Fix errors with Language downloader and static/dynamic file names

The Language Downloader should be able now to correctly manage both
dynamic and static file requests.
Fix issues with extracting data from multiple files (part files)

- Fixed some errors when references to config.source in the code
was not handled correctly in case of regular expressions (errors
reported by @jpluorange in #90)
- Add comments to the default download and extraction config files
to clarify when and how to use regular expressions
Merge pull request #117 from ninniuz/fix_multiple_files_download_extraction

Fix issues with extracting data from multiple files (part files)
Merge pull request #116 from ninniuz/113_language_downloader_fix

Issue #113: Fix errors with Language downloader and static/dynamic file names