AWS regions

I believe there has been some misunderstanding what EC2 zone, region, placement, and placement group are.

1. A *region* is one of us-east-1, us-west-1, us-west-2, etc. It is roughly a city (a server facility) where the server is located.
2. An *availability zone* is one of us-east-1b, us-west-2a, eu-west-1b, etc. It marks a unit within a region.
3. *Placement* is a synonym for availability zone. It is used in the EC2 API.
4. *Placement group* is a completely different concept from placement (unfortunately). It marks computers which are somewhat close to each other in EC Compute Cloud (HPC).

See the AWS documentation to read more about [regions, availability zones](http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-regions-availability-zones.html#using-regions-availability-zones-setup), and [placement groups](http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using_cluster_computing.html).

This patch adds AWS regions and:

* keeps placement groups introduced in the pull request #9,
* enables availability zones disabled in the pull request #9,
* keeps the default behavior: select a random zone within the us-east-1 region, and
* fixes the documentation changes introduced in the pull request #11.