[bc break] Remove "Selenium" dependency

[aik099]

Currently the Selenium dependency is hardcoded:

• the selenium Mink driver is a requirement in composer.json
• the timeout and desiredCapabilities settings in BrowserConfiguraiton only apply to Selenium

After #12 is implemented we can, but this will be BC break for sure:

• replace selenium requirement in composer.json with suggest tag, that Mink itself uses
• move desiredCapabilities and timeout from global BrowserConfiguration options to Selenium2 driver specific options
• set empty default driver
• upon attempt to create driver without actual driver name set throw an exception