add rect to FrameTree
add comment re: clipping rect
placeholders for FrameRectMsg from Layout to Constellation
add rect to FrameTree
added CompositorLayer::from_frame_tree
reorganized constellation.
compositor routes dom events via constellation.
constellation handles iframe sizing and resizing.
refactor iframe element fields to support sending size to the constellation
support in layout to send iframe sizes
removed old TODO
add iframe support
some more minor refactoring in constellation
Better layer resize methods
move navigation_type from Pipeline to FrameChange
Implement epochs; fix integration bugs
fixed bug in FrameTree.replace_child that caused a copy that broke the code
Add Epoch newtype; address review comments
add todos for iframe size messaging in float/inline
remove commented out code from compositor_layer::from_frame_tree
auto merge of #702 : tikue/servo/master, r=pcwalton

This is a hodgepodge of changes necessary to get iframes working. With this PR, iframes work in some cases, if CSS width and height style properties are set for the iframe and the iframe is in a block (inline and float are not supported yet).