Extractor supports .tgz archives

Signed-off-by: Aram Price <aprice@pivotallabs.com>
Extractor is not responsible for removing archive

Signed-off-by: Aram Price <aprice@pivotallabs.com>
Use gofileutils instead of shelling out to `cp`

Signed-off-by: Aram Price <aprice@pivotallabs.com>