Thoughts on v2.0

Breaking changes it would be nice to make on iron-ajax if/when we do:

• remove dependency on Promise polyfill, it's the only one of our elements that depends on it, and it's just for the imperative API.
• rename error event to not clash with window.onerror.
  • can revisit the decision on whether iron-ajax's events should bubble, #156 presents one good use case in favor of bubbling
• consider making the core logic a behavior, so that users can easily customize / override stuff like parameter encoding and POST body encoding