Twiddle text node to trigger ASAP flush

Some buggy `MutationObserver` implementations or polyfills may not trigger a mutation when an attribute is just set again to the same value.

This text node twiddle approach is based on Polymer's [microtask.js](https://github.com/Polymer/platform-dev/blob/master/src/microtask.js).

Fixes #170

Related

* https://github.com/tildeio/rsvp.js/issues/170
* https://github.com/jakearchibald/ES6-Promises/pull/2

/cc @jakearchibald @azakus