Added Database Class, and method's to do database connection in MySQL

I've created one Database class that is more general and so far is
resposable to make a connection with the database.
I've also created one Database_MYSQLI class that inherit from the
Database and got some specific method's responsables to make a
connection with MySQL databases.
And there is a new configuration file called database.php that keep's
the database settings.

There is a new method in the Loader class, called database, and
all starts in this method. The method work's in a way that is possible
to have multiple database settings and switch to them with only one
little change in the configuration file.

All starts with the method loading the database settings located
in the config/database.php file. After that he loads the definition
of the Database class, then, based on the settings defined in the
configuration file, he load's the proper specific database class,
for example, the Database_MYSQLI class.

After he loaded the right class based on the configuration, he will
create an object and call the method initialize of that class, that
will stablish the connection with the database. The object created
is assigned to the Controller object, so it can be accessed in any
controller, and in any model in the application.