Fix (attr) iterators to return references to real objects, not temporary proxies

[vslavik]

Attributes iterators, in particular, return a reference to an internal attr object owned by the iterator. This means that the reference is only valid for the lifetime of the iterator and code like this is invalid:

xml::attributes::attr& a = *(root.get_attributes().begin());
// iterator freed, including its attr object, 'a' no longer valid
a.set_namespace(xml::namespaces::ns("href2", "p2"));

This is extremely unexpected for a C++ programmer.