Rewrite runloop guide autorun FAQs

A rewrite of the autorun FAQs in the Runloop guide.

Fixes #1712 #1713 Partially addresses #1390
I _think_ I understand what is happening. The [current wording](http://emberjs.com/guides/understanding-ember/run-loop/#toc_what-happens-if-i-forget-to-start-a-run-loop-in-an-async-handler) on the site implies that Ember will wrap all the code in an async handler in an autorun. I will make another pass at this.