add subscript to RXMLElement
add support to set attribute
add append and prepend
add Remove attribute
Add empty() and removeChildren()
add remove()
add setTag method to element
add parent method to RXMLElement
cached element, so that only create one Objective-C object for one node
add more test for common parent
add nextSibling and previousSibling method
remove nodes should remove the cache from doc holder
remove append and prepend, add addChild, addNextSibling and addPreviousSibling
add support for provide encoding and options to HTTP Parser