option for caller ID via /me command

[mndrix]

Many XMPP clients have poor support for XEP-0172 User Nicknames and XEP-0054 vcard-temp which we use for caller ID. In some cases they lack support completely. In other cases the UI is bad. It would be nice to be able to fall back to XEP-0245 /me Command as a last effort to communicate this information. Since /me is handled via normal chat messages, it should work with every XMPP client.

Support should be hidden behind an option (default off) because it generates message content which the SMS user didn't create themselves. Many users might not want that sms-over-xmpp injecting content into their chat stream.

• track time of most recent message to/from a contact (new fields in xmppContact)
• add "caller-id./me = yes" option
• send "/me is John Wayne" as first message if no previous message exists

This is a follow up to #9. It would benefit from the longevity described in #19

[mndrix]

In my day-to-day usage, caller ID using other mechanisms has been poor. It's really awkward with Swift and Adium and unreliable with Conversations. Having a reliable fall back like /me would be nice.