Async snippet early AM draft [freaky friday fun]

Not ready, want comments. @shane-tomlinson @seanmonstar @lloyd thoughts here?

I woke up thinking about the need for an async snippet this morning, maybe it's all the velocity conf tweets I've been reading, dunno ;-) Anyway, it's working locally for me.

This could go a number of different ways:

* it could grow into a self-hostable include.js
* it could just be a fancy async snippet with some stuff hidden behind the scenes + detailed docs, a la google analytics' [async snippet](https://developers.google.com/analytics/devguides/collection/gajs/)
* it could grow into an expert-mode async SDK (but that would change our API which is a huge decision, not to undertake lightly).

Thoughts/comments/heckling appreciated