Refactored getValue to use the webdriver API rather than a big JS script

This uses the WebDriver API to get the value of a field instead of building a huge JS script to handle it. The general case to get the value through WebDriver is simple: ``$element->attribute('value');``. But we need to handle a few special cases:

- checkboxes. Mink defines the return value of ``getValue`` on a checkbox as a boolean checking whether they are checked (so whether they will be submitted), not as the submitted value when checked (to get it, we need to use ``$checkbox->getAttribute('value')``). So this does not match the WebDriver API.
- radio input. Mink defines the return value of ``getValue`` on a radio input as the value of the selected radio input with the same name, not as the ``value`` attribute of the current radio input. I decided to keep the JS script here (but much more readable as it handles only this case) as doing the same through WebDriver would require many API calls.
- select multiple. The WebDriver call ``$element->attribute('value');`` does not support multiple selects. It only returns 1 selected option, not an array of all selected options.