Allow passing a method, request body, and headers to the pipeline in LoadUrlMsg
auto merge of #3626 : Manishearth/servo/form-prep, r=jdm

Framework for form submission
Disable failing reftest (#3636)
auto merge of #3637 : mbrubeck/servo/disable-link-style-reftest, r=pcwalton
Factors out DOM traversal, keeping the code in `parallel` free of traversal-specific logic.

DOM traversals and Flow traversals look very similar. This patch unifies them
with the preorder/postorder pattern. Hopefully, it also opens the door for writing
the traversal code only once, instead of the duplication we have today.