Update to support more regions

Relates https://github.com/hashicorp/terraform/issues/524
Added `eu-central-1`, `cn-north-1` and `us-gove-west-1`(hi NSA)
I checked `goamz`, @mitchellh already has added support for those regions
Merge pull request #525 from pmoust/patch-2

provider/aws: update to support more regions