/whisper name message

the idea is to emulate IRC's /notify functionality

You could write

    /whisper Destiny my skype name is hunter2

and only Destiny would get the message.

Whispers are distinct from future Private Messages, because they show up inline with the main chat, and they don't get saved to the database. (Whispers piggyback on the MSG code, while PRIVMSG will piggyback on chat conversations/rooms/channels).

TODO:
- have someone else review the code
- refactor to make the code more "conventional" -- this is literally the first golang thing i've written
- test with the frontend code to make sure it plays nicely