No more dependencies, clean up native form mirroring, publish to npm

This is a huge release with some substantial changes, though the breaking changes noticeable to users are likely very minimal.

1. Ajax-form no longer has any hard dependencies, not even Polymer! Ajax-form 2.0 is a pure web component able to stand on its own, and Polymer is no longer a dependency. Note that you may still use Polymer and ajax-form in your project, if you wish. Also, Polymer's custom element form fields continue to be supported as well, provided they behave like native form fields (which they should)
2. An attempt was made to make ajax-form a bit more element-like. For example, previously, if a method attribute was not included, an Error was thrown. 2.0 is taking a different approach to make ajax-form seem a bit more native. If you don't include a method attribute in your form, a default of "GET" will be used.
3. A bug that prevented empty text fields from being sent with the submit request has been fixed.
4. Ajax-form is now on npm! In fact, this is definitely the preferred way to download ajax-form going forward.
5. You can now prevent a form from being submitted simply by preventing the default action of the "submitting" event.
6. 2.0.0 brings support for "grouped" form fields. That is, multiple form fields that share the same name attribute will all be included in the submit request for all enctypes, even "application/json"!

To download ajax-form, simply npm install ajax-form.