Use pure JavaScript code for node attribute retrieval

Currently used `attribute` method of `WebDriver` works differently for `input` and `non-input` tags, when it comes to missing attribute querying (e.g. missing attribute value on `non-input` tag is returned as empty string, but for `input` tag `null` is returned instead). Therefore some workaround code was added, that considers empty attribute value as `null`, which in turn breaks whole logic of `hasAttribute` method (see https://github.com/Behat/Mink/issues/389).

This happens due fact, that according to some magic, that `WebDriver` is doing behind the scenes (see http://selenium.googlecode.com/svn/trunk/docs/api/java/org/openqa/selenium/WebElement.html#getAttribute(java.lang.String)) we might get JavaScript property value with same name instead of an expected attribute value.

Therefore I'm proposing to use pure `getAttribute` method of HTML nodes from JavaScript for node attribute retrieval.