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 option can also be specified for a mailbox.
- Implement some of the pending Gmail::Message specs to verify that the
  read-only option is respected.
Merge pull request #6 from Yesware/read-only-option

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