Add debug filter

[jlebrech]

Something like this:

PolymerExpressions.prototype.debug = function (value) {
  return JSON.stringify(value);
}

I followed your pattern to add this to my app, as i'm working with a third party api and need to inspect the responses, and this was quite useful.

AngularJS has a filter like this which is handy,

[addyosmani]

Basically this filter, right? https://github.com/angular/angular.js/blob/master/src/ng/filter/filters.js#L480

Would you like to file a PR for this change?. If not, I can get it added as part of the next release.