Several improvements

There are some improvements:
* Easier to configure: Just one file for database configuration and for changing the default browser. All settings should be configured in just one file.
* Better database abstraction. Now it's easier to change driver (mysql vs mysqli), with time I'll consider to abstract with PDO or maybe something lighter. This still needs to be improved so it may use different database engines.
* Some sql code injections preventions. As none input was escaped in the original code.
* Better way to check if an input was used. There's a difference between '($_GET['var'])' and 'isset($_GET['var'])'.
* Other minor changes.