* add option for specifying Mink driver as part of the browser configuration
* set driver to 'selenium2' to preserve backwards compatibility
* add driver factories for Selenium2, Zombie, Goutte and Sahi drivers (only basic options supported)
* allows to register additional Mink drivers

For example, to configure a test to run with the Goutte driver, use

public static $browsers = array(
	array(
		'driver' => 'goutte',
    )
);

There also is a new configuration option 'driverOptions' which can be used by driver factories when instantiating the Mink driver.
It defaults to an empty arrray, places no restriction on the data structure and is currently not used by any of the driver factories.

1. Extracting driver factory registry into separate `DriverFactoryRegistry` class
2. Simplifying architecture by allowing `BrowserConfiguration` itself to create driver via new `createDriver` method, that uses above create factory to acquire proper driver factory
3. Eliminate getter methods for `BrowserConfiguration` parameters by using "__call" method
4. don't allow internal `type` `BrowserConfiguration` parameter to leave the `BrowserConfigurationFactory`
5. the `BrowserConfiguration::getType` method wasn't covered
6. allow DEV version of Mink to be used for testing purposes
7. allow each driver (via it's factory) to provide defaults for browser configuration
8. the "api_username" and "api_key" browser configuration parameters renamed to "apiUsername" and "apiKey" for consistency, but no BC break because old parameter names still work
9. updated documentation to include description of `driver` and `driverOptions` parameters