Moved AWS code into AWS namespace

Signed-off-by: Colin Humphreys <chumphreys@pivotallabs.com>
Moved Bosh::Stemcell::AwsRegistry to Bosh::Stemcell::Aws::Region

Signed-off-by: Colin Humphreys <chumphreys@pivotallabs.com>
Make spec safer

Signed-off-by: Colin Humphreys <chumphreys@pivotallabs.com>
Ami no longer middle-man between light stemcell and region

Signed-off-by: Colin Humphreys <chumphreys@pivotallabs.com>
Encapsulate default region in Region class

Signed-off-by: Colin Humphreys <chumphreys@pivotallabs.com>
Rename Bosh::Stemcell::Stemcell to Bosh::Stemcell::Archive

Signed-off-by: Colin Humphreys <chumphreys@pivotallabs.com>