All strings must be XML compatible: Unicode or ASCII, no NULL bytes or control characters

Hi,

I had this error today. I actually introduced the illegal character and fixed the problem at the input level. I'm just wondering if it should be handled by the lib though...

``` python
import html5lib

src = u"\x01"

parser = html5lib.HTMLParser(tree=html5lib.treebuilders.getTreeBuilder("lxml"), namespaceHTMLElements=False)
tree = parser.parseFragment(src, container="div")
```
