Bugfix: sax is now able to handle a tag with the name "xmlns".

Before this patch, sax-js would produce incorrect events if it had to handle a tag named ``xmlns`` in the input. Something like:

```
<xmlns/>
``` 

would trip it up. The local name would be the empty string and the prefix would be set to ``xmlns``. I've modified the code so that this kind of transformation now happens only for attributes.

For the record, I came across this issue while running sax-js over a test suite designed by James Clark for Relax NG validators. I don't know what possessed him to test for ``<xmlns/>`` (maybe the same bug in another piece of software?), but here we are.