Move script local_data_pop out of dtor

Fixes #570, which was caused by a second reentrant call to the dtor.
Fix flow tree debug printing.

Nested `debug!()` calls are no-ops, so change it now does a `println`. Also
moved to the `debug!()` calls out of the profiled block.
Float children are now positioned relative to the float's box.
Add a reftest for relative widths.
auto merge of #612 : metajack/servo/float-children, r=jdm

They were being positioned outside their parent's box in some cases.
Bump layout data ref count in unsafe_set_layout_data

We need this or else we double-free the layout data box and crash.
auto merge of #613 : metajack/servo/basic-width-reftest, r=pcwalton
auto merge of #608 : kmcallister/servo/script-exit, r=metajack
Fix calculation of float height and width.

We were overshooting on height by including margins, and undershooting on
width by not including margins, borders, and padding.
auto merge of #614 : metajack/servo/float-height-width, r=pcwalton

We were overshooting on height by including margins, and undershooting on
width by not including margins, borders, and padding.
Prettify make output during builds.
auto merge of #615 : metajack/servo/pretty-make, r=jdm