Provide better usage help for users

This should fix #3.

Some examples:

The first user to run `/pair` by itself will see:
> No one up for pairing (yet!). Pair up, yo.
> Usage:     /pair yes|ok|no <what you want to do with fun people>  (or just "/pair" to see the list)

Then, running `/pair yes` will set the user's status, display it, and display the usage help again. 
> Set your pairing status to: yes
> Yes! Someone should come find me now. Let's pair:
>    techieshark: 
> ---------------------- Pair up, yo. (Go find 'em!) ----------------------
> Usage:     /pair yes|ok|no <what you want to do with fun people>  (or just "/pair" to see the list)

Note that there is no `/pair help`. Having a `help` command only works for people who would know how to look for that, and since the usage is just one line anyway, we might as well share that.