Remove jQuery by using Promise and XMLHTTPRequest

This removes the need to include jQuery and uses the new `Promise` in ECMAScript 6 (with link to a polyfill).

fixes #29, refs #19

TODO:

- [x] make sure it works in node
- [x] maybe add `.done()` and `.fail()` to `Promise` so the rest of [oerpub/github-bookeditor](https://github.com/oerpub/github-bookeditor) works
- [x] add [jakearchibald/es6-promise](https://github.com/jakearchibald/es6-promise) to `README.md` (or include it in the build?)

@Pomax and @hyspace: any thoughts?