A small collection of changes and code cleanup to successfully run
'corso repo init s3' manually. Adds a default s3 url (might want
this configurable for local testing) and aws session token support.
* introduces storage pkg (#25)
Adds a new package /pkg/storage. Storage is used to
communicate storage provider information throughout the corso
app. In particular, it allows per-provider data to cross interface
boundaries without slicing their details.
* use consts for s3 config keys
