added shallowClone to Term

This will help shallowClone a term whenever necessare.
Adding clone to the NodeData

The operator of the term was being modified, and this was modifying the
original style declaration. So instead of modifying the original term
form the declaration we make a shallow clone of that term and modify it.
This will not modify the original style sheet.
Merge pull request #4 from UprootLabs/fix/line-height

Fix/line height
Allow element.getAttribute() to return null
Fix background-position processing to report error correctly
Recovery in declarations revised