Add well-known filesystem abstraction package 'wkfs' for cloud deployment ease.

Part of Camlistore deploy to GCE easily. Will use paths like /gcs/bucket/foo or /gce-meta/attr for some
things previously-assumed to only be stored on the OS filesystem.

Change-Id: I2ed6cf5759c4d277f4e89a33a0fcfeee8bcbbf82