Added the skeleton for a read-only file implementation.
Added the skeleton for a read-only file implementation.

jacobsa/timeutil,kahing/gcsfuse,BanzaiMan/gcsfuse,mbrukman/gcsfuse,GoogleCloudPlatform/gcsfuse,GoogleCloudPlatform/gcsfuse,jacobsa/ratelimit,GoogleCloudPlatform/gcsfuse,BanzaiMan/gcsfuse