make it work when there's a dot in the attribute selector

Hi Wouter,

Thank you for making this htmlparser. But I got a issue when using attribute selector "[src='aaa.jpg']" to select a \<img\> node. It seems that the dot in the jpg file caused it. So I try to work around this issue. 

If you think that my code style is bad, please feel free to change the code. But I really want this feature supported in this library.

Thanks,
Rocky