Add getDefaultDatabaseDriver() to DatabaseManager

Currently, naming database connections anything but their driver name sometimes fails (for instance when using OctoberCMS) because it's easy to use getDefaultConnection() to get the connection name but harder to drill down to the default connection's driver, so this shortcut is often used.

This change makes it much simpler and more realistic for database config files to be named semantically going forward, for instance:
``` php
return array( 
	'fetch' => PDO::FETCH_CLASS, 
	'default' => 'production', 
	'connections' => array( 
		...
		'production' => array( 
			'driver'   => 'pgsql', 
		...
```
instead of being confined to:
``` php
return array(
	'fetch' => PDO::FETCH_CLASS,
	'default' => 'pgsql',
	'connections' => array(
		...
		'pgsql' => array(
			'driver'   => 'pgsql',
		...
```