Update 2014-03-06-balanceds-architecture.md

In terms of security, I agree that this is probably high level enough that we should avoid getting bogged down in implementation detail.

However, I think it might be good to emphasize the advantages of 

1. keeping the PCI store dumb and dead simple and 

2. isolating it from the rest of the API to the point that we can build features on the API and expose the API to more developers without worrying too much about messing with or otherwise exposing the PCI Store.

And I guess we'll eventually follow up with a post on transaction level security. Weeee!!
Merge Tim's changes.