added subpage ids to map from iframe to all associated pipelines
current frame tree is copied upon subframe navigation to prevent changes from being transitively received
when loading an iframe, now searches for all instances of parent pipeline in the navigation context to append the child frame to
add next_subpage_id to Page struct
fix how FrameTrees are copied
refactor constellation methods to follow FP style
auto merge of #647 : tkuehn/servo/master, r=metajack

This will be necessary for the constellation to properly assign sizes to iframes when a parent layout performs reflow.