Fixing compiler typecasting warning.

Warning was as follows:

../../../../ext/nokogiri/xml_node.c:17:14: warning: incompatible pointer types initializing 'xmlNodePtr' (aka 'struct _xmlNode *') with
      an expression of type 'struct _xmlDoc *' [-Wincompatible-pointer-types]
  xmlNodePtr doc = node->doc;
             ^     ~~~~~~~~~
Rename C functions to reflect getter/setter roles
Add Node#lang and Node#lang=
Merge branch 'flavorjones-setlang'
DocumentFragment delegates #errors to its document

It's just confusing to maintain an independent errors array.
Whitespace and test cleanup.
Ensuring error handling in HTML docs and fragments

Specifically:
- when copying a node between documents, errors are propagated to the
  destination document
- when duplicating a node, silence errors

Fixes #1196