First pass at CompiledPackageDownloader

Signed-off-by: Jesse Zhang <jz@pivotallabs.com>
Include stemcell sha1 and package fingerprint

in the exported packages yml

Signed-off-by: Jesse Zhang <jz@pivotallabs.com>
Fix archive path

Signed-off-by: Jesse Zhang <jz@pivotallabs.com>
correct spec description
spin out the blobstore client and closing file expectations
spin out the downloaded blobs tests