Add the ability to log Drive Changes API responses
Guard write ops in readOnly, + remaining write ops

- Guard each of the existing write operations when the readOnly option
  is set.  This looks to be necessary because when implementing
  fuse.ReadOnly() in bazil.org/fuse, I discovered that osxfuse has no
  option to mount a Fuse FS read only.  :-/
- Add a TODO skeleton for each of the remaining write ops, to make sure
  they get the readOnly guard while I'm thinking about it.
Merge commit 'f367d01d25a973a541474e9f9a00c70ddd7f8bb2'

Conflicts:
	drive_db/drive_db.go
Flesh out README, clean up package doc strings