use Display, not custom to_string, for Mood

[warner]

@copyninja mentioned (in warner@46d31b8#commitcomment-29139497) that to_string should be part of the standard Display trait, but I used it as a standalone method on the Mood struct. I'm up for making this Display, but the specific strings used there are part of the wire protocol, so they're fixed and should not be changed in the future (e.g. as part of i18n language-specific translations). So we need some comments to that effect, and some unit tests that ensure the strings remain fixed.

[warner]

I implemented Display for the newtypes from #32, and ran into just this sort of problem:

• Mailboxes were stringified as Mailbox(abc123) instead of just abc123, which made its way into the OPEN and CLOSE messages we sent to the server, which confused things
• the Moods are named Happy and Scared and Error, but in the CLOSE message they're spelled happy and scary and errory, which would break the server statistics

I added unit tests on the message serialization code to make sure these are getting stringified properly.