* validate required storage props (#85)
Centralizes validation of required storage config properties within
the storage package. Requiremens are checked eagerly at
configuration creation, and lazily at config retrieval.
Additionally, updates /pkg/storage tests to use suites
and assert funcs.
* add validation failure tests to storage
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.
* adds kopia pkg to handle integration (#25)
internal/kopia will be used to abstract all kopia integration in a
central location.
* defer blob.Storage closure in kopia
blob.Storage objects must get closed at the end of their usage.
This currently isn't getting called.
