Change asap mutation observer implementation to touch text node

Alternative PR to #1 as suggested by @azakus.

Opposed to setting attribute to the same value which may not trigger a mutation on buggy browsers or with a MutationObserver polyfill.

Hat tip to Polymer microtask.js
https://github.com/Polymer/platform-dev/blob/master/src/microtask.js

/cc @jakearchibald @azakus