Added a --read_only flag to gcsfuse.
Added a --read_only flag to gcsfuse.

Fixes #48.

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