Rename the element field of the DOM Node hierarchy

Renamed htmlelement, and element

Fixes #924
Rename the node field of the DOM node hierarchy

Renamed the Node elements

Fixes #924
auto merge of #959 : luisbg/servo/master, r=jdm

Issue #924
deactive profiler when not in use; use newtype structs for task chans
workaround for broken cross-crate generic newtype structs

add issue number for newtype struct issue
cleaned up code in constellation to make it more maintainable