Added hostname support. Added ability to change database in config.

I've added hostname option for database structure load/dump functions. Early it was a problem when database was placed on other host.
Also. I've added ability to change database name in config file. It's helpful if we use diffretd database profiles for different environments. For example, migrations config can be like this:
```php
return array(
    'log'      => NULL,
    'path'     => DOCROOT . 'migrations',
    'database' => Kohana::$environment
);
```