Added DocumentFragment, fixed API-doc for document.create<Type> methods

lib/dom/document.js
*  fixed return values for all create<Type> methods (from the incorrect 'void' to <Type> object
+ added createDocumentFragment method

lib/dom/documentfragment.js
+ added DocumentFragment module (extends Node)

lib/dom/node.js
* refactored the inner mechanics of both appendChild and insertBefore to correctly implement the mechanics of working with DocumentFragments and simplify the meddling with child nodes

lib/simple/factory.js
+ added the document fragment type

test/documentfragment.js
+ added tests for DocumentFragment-specific behavior