Refactor node/attribute tree operations

All ad-hoc attribute operations are now implemented as explicit low-level
functions, and xml_node just uses them. Additionally extract commonly used
is_attribute_of and move detaching of node from append_node to remove_node
- append_node now only works on detached nodes (small increase in parsing
  performance).

git-svn-id: https://pugixml.googlecode.com/svn/trunk@1056 99668b35-9821-0410-8761-19e4c4f06640