Add xpath_node_set::iterator that is the same as const_iterator.

Exposing true mutable iterators allows the user to violate sorting order
contract. However, some generic algorithms (i.e. Boost ForEach) require
iterator methods to be present.

git-svn-id: https://pugixml.googlecode.com/svn/trunk@998 99668b35-9821-0410-8761-19e4c4f06640