Create a basic jsdoc externs file for Polymer
Add a few more methods on PolymerElement
Add a license and an extern for the fire method

This change is by jlklein@google.com
externs: attributeChanged, Element -> Node, !null 

Addressing comments in the pull request.

A couple more require more experimenting, followup commit coming.
externs: whitespace
externs: add cancelAsync; improve async
externs: add type for onMutation callback
Merge pull request #769 from rictic/patch-1

Create a basic jsdoc externs file for Polymer