Adds possiblity to select different databases.

This currently enables MySQL, MariaDB and PostgreSQL. 

This fixes #74

Personally I don't like the ```at``` method of phpunit as it makes the tests really fragile.

Can we use a callback to return the value based on the request parameter and then verify that the values where correctly assigned?

Something like

```php
$this->request->expects($this->any())
    ->method('get')
   ->will($this->returnCallback(function($param) {
      $params = array(
                'dbserver' => 'pgsql',
               'user' => 'user',
               // etc.
       );
       
       return isset($params[$param]) ? $params[$param] : null;
   })
);

// $this->assertCorrectvaluesAreAssigned() somehow
```
I've done it from memory so maybe ```returnCallback``` is not the correct method here