Prefer single quotes for uninterpolated strings

Signed-off-by: Abhijit Hiremagalur <abhi@pivotallabs.com>
Introduce PublicStemcell

Encapuslates the properties and behaviour associated with a public stemcell from the perspective of the CLI
Introduce PublicStemcellIndex

Encapuslates interaction with the public stemcell index
Introduce DownloadWithProgress

Downloads a file to the current directory, reporting progress along the way
Introduce PublicStemcellPresenter

Encapsulates public stemcell ux independantly of the cli