Add option to specify read-only access for a Gmail::Mailbox

- Previously, all access to mailboxes was read-write (SELECT) with no
  option to override the behavior.
- This change adds a `:read_only` option (default: false) to `Gmail::Client`.
  The value of that option is used as the default for mailboxes accessed
  using that client.
- The read_only can also be set for a specific `Gmail::Mailbox`.
- Implement some of the pending `Gmail::Message` specs to verify that the
  read-only option is respected.