Add `CompositorTask::create()` to initialize and start CompositorTask.
auto merge of #1419 : saneyuki/servo/create, r=pcwalton

This hide initializing steps of `CompositorTask` to make it easy to handle variables which related to compositor code.

Related: #1351.
Added recognition of positioned offsets

  top, bottom, right, and left
auto merge of #1407 : ibnc/servo/positioned_box_offsets, r=metajack

For #1406

Not really sure about some of my naming conventions, but offset was kinda taken in box.rs :/

Also this doesn't fully implement the offsets, but I'll do that alongside #782.