Correct some of the misconceptions about how to run Riak in EC2.

I blew innumerable hours trying to set up a VPC environment for my Riak cluster, then ran into serious problems with my other systems there (Riak was fine!) - so I finally went back to Classic EC2, and worked out some of the kinks with how to do that in a way that makes sense and is durable.

I've tried to take a first pass at modifying the documentation to match with what I've discovered.

One point I'm really curious about - the "binding to 0.0.0.0" concept - I noticed it's not featured at all in the documentation, but the Riak AMI seems to do it. I've mentioned it at least briefly in the documentation, but I figured either:

a) It's a bad idea - if so, we should talk about why.
or 
b) It's totally fine - in which case, why doesn't this feature more prominently?

I can totally get why the default install doesn't open itself up (binding to localhost) - Riak open to the world would *not* be good.