Fixed Wordpress import when importing WXR files above version 1.1.

When the WXR version is >1.1, the first loop iteration will try with a non-existent `wpns` and `item.find('%spost_type' % wpns)` is None, hence `.text` will raise an exception.

The WXR that I exported has all types of items, hence it's more flexible to assign the post type, then filter for `post` and `page` types later.