Denney check in

Closes #7.  

-Debated whether to add a CHECKOUT user action instead of updating the end_date on the CHECKIN user log.  Went for the latter.
-For checking out, I probably also need to think about whether a POST to checkout actually makes sense verus PUT to checkin.

But, it works!

@jnm Would you be interested in making a view of all the people checked in on the current day for admins?