More flexible handling of attributes

We're using Nori to work with a SOAP webservice, and we've run into a few places where it wasn't quite as flexible as needed. These commits address our needs while trying to stay consistent with the current API and design.

* There is a new global setting `Nori.convert_attributes` which works just like `Nori.convert_tags`, only for attributes. The block is expected to take 2 arguments (key and value).
* When `Nori.strip_namespaces` is true, also strip namespaces from attributes.