This implements the DOM tree accessors that return a HTMLCollection
Fix some implementations for DOM tree accessors
Factor out CSS length to Au conversion
auto merge of #659 : sonwow/servo/dom, r=jdm

Basic implementation about DOM tree accessors in HTMLDocument.
(image, embeds, plugins, links, forms, scripts, anchors, applets)
auto merge of #663 : sanxiyn/servo/from-length, r=metajack
Simplify HTMLCollection predicates
auto merge of #666 : kmcallister/servo/htmldocument, r=jdm
Move webidl files to dom/webidl
Modify BindingGen.py(add outputFolder)
Update Makefile.in
Add bindings-test(in Makefile.in)