Improve screenshot system

Hi Sebastian

This patch add more flexibility in the configuration of the screenshot.

Currently the configuration of the capture screenshot is made in the SeleniumTestCase class with the attributes
$captureScreenshotOnFailure, $screenshotPath, and $screenshotUrl.
With this solution it is not possible to define screenshot path or host for each OS.

With this patch the configuration is now made in the PHPUnit_Extensions_SeleniumTestCase_Driver class.
E.g.
array(
        'name'    => 'Safari on Windows XP',
        'browser' => '*custom C:\Program Files\Safari\Safari.exe -url',
        'host'    => 'my.windowsxp.box',
        'port'    => 4444,
        'timeout' => 30000,
        'captureScreenshotOnFailure' => true,
        'screenshotPath'	=> 'c:\Temp',
        'screenshotUrl'		=> 'http://my.windowsxp.box/screenshot/safari/'
	'captureMethod'	=> 'captureEntirePageScreenshot'
      )

If you want to discuss this change with me, I have the same nickname in Twitter