Some improvements.

- File_Downloader now keeps track of what it had downloaded.
- Some bug fixes and refactorings.
BookDb is added.

- A new database and data model is added.
- Every file is checked for its type after download.
- Insertions are now done in transactions.
- Config file is added.
Some clean-up.

- README file is updated with file information.
- Renamed .sql files to increase consistency.
- Ignore config and database files.