New and updated methods.

* New set_attribute() method for setting individual attribute
* New encode() method for making strings XML safe
* Updated set_attributes() method now loops though attributes
and uses set_attribute() for each
Use \DOMElement instead of DOMNode & correct setting headers

Use DOMElement instead of DOMNode for arguments, and use \ for namespace
Fix setting headers using [] = $ instead of [] => $
New class XML_Node

Class for creating and working with DOMElements
Now extends \DOMElement instead of resources\XML_Document

Rerite all methods to adapt for these changes
Document body is a resource\XML_Node

Update set method and all similar to use methods for XML_Node instead of
DOMDocument
Added/updated documentation
More documentation updates