Create a basic jsdoc externs file for Polymer

My preference would be to punt on this for the moment, as the existing use cases I've seen for compilation would put the Polymer() calls far away and uncorrelated with the element declaration such that I suspect that it will not be possible to infer the name.

(ok, that and I don't know how to set up the types such that Polymer('hello-world', {}) and Polymer({}) are both accepted but Polymer({}, {}) is rejected)