Ignore attributes with illegal characters in name

Hi,
I would like to fix the issue #23 ASAP, because I parse a lot of pages that throw `DOMException` related to it. I read @technosophos's [opinion](https://github.com/Masterminds/html5-php/issues/23#issuecomment-35008560) about the issue but I think there's nothing to think about because of current `DOMElement`'s implementation in PHP. Reality is that the method `DOMElement::setAttribute` throws an exception so element with the attributes like `?`, `;`, ... can't be created. That's why I implemented the other possible way - parser ignores them.

All I want is HTML parser that doesn't throw exceptions and believe we discuss to the proper solution :)