Added command log() and logTypes() for Selenium2TestCase

This commands needed for retrieve JavaScript errors from WebDriver, look at JsonWireProtocol: https://code.google.com/p/selenium/wiki/JsonWireProtocol#/session/:sessionId/log
Just for simple example:
```php
class WebTest extends PHPUnit_Extensions_Selenium2TestCase
{
	protected function setUp()
	{
		$this->setBrowser('chrome');
		$this->setBrowserUrl('http://stuff-dharrya.rhcloud.com/get_js_error');
	}

	public function testTitle()
	{
		$this->url('http://stuff-dharrya.rhcloud.com/get_js_error');
		print_r($this->logTypes());
		print_r($this->log('browser'));
	}
}
```
Will output:
```bash
Array
(
    [0] => browser
    [1] => client
    [2] => server
)
Array
(
    [0] => Array
        (
            [message] => http://stuff-dharrya.rhcloud.com/get_js_error 9 Uncaught ReferenceError: someVariable is not defined
            [timestamp] => 1376507135656
            [level] => SEVERE
        )
)
```