Updating custom element callbacks to spec
README: missed a few readyCallback updates
s/Loader/PolymerLoader
freshen tools
Loader -> PolymerLoader
update callback names to match spec, make tests compatible with native document.register
CustomElements.upgrade needs a stub under native
remove HTMLElementElement implementation
fix/update tests
support type-extension form of document.createElement
Merge pull request #54 from Polymer/master

8/15 master -> stable