Merge pull request #3 from felixge/master

Features: RemoveElement and RemoveAttr
Merged from Felix Geisendörfer's CL.
Added RemoveAttrByKey and RemoveAttrByKeyFull.
Add AUTHORS file.
Added document and element Copy() functions.
Allows deep (recursive) copies of element trees.
RemoveElement and RemoveAttrByKey tweaks.
RemoveElement clears the removed element's parent pointer.
RemoveAttrByKey APIs return a pointer to (a copy of) the removed attribute.
Implement Root method