Removing "setContainer" and "getContainer" methods

1. removing "setContainer" and "getContainer" methods, because element knowledge of his surroundings is not it's business
2. updating "LabeledElement::getLabel" method for finding corresponding label by "for" attribute based on checkbox/radio itself and not it's container
3. fixing minor HTML markup errors in Live tests index.html
4. updating index.html to ensure, that <label for="..."> standing before checkbox/radio is also found

Related to Behat/Mink#593
Closes #82