public.json: Add 'POST /drops' to create a new drop

The posted data should match the usual 'drop' schema except that you
shouldn't be setting the 'id' property.  I haven't been able to figure
out a reasonable Swagger syntax for that though [1].

[1]: https://groups.google.com/d/msg/swagger-swaggersocket/i7EEXw_MQZ8/wcXXsQa2CYAJ
     From: "W. Trevor King" <wking@tremily.us>
     To: swagger-swaggersocket@googlegroups.com
     Subject: Requiring pet.id but disallowing newPet.id (was: Altering a model)
     Date: Fri, 19 Dec 2014 14:16:04 -0800
     Message-ID: <20141219221604.GF19920@odin.tremily.us>
public.json: Add 'PUT /drop/{id}' to update a drop

The data should match the usual 'drop' schema.  In this case, I don't
mind if folks set the 'id' property or not (whichever is easiest), but
if they set the 'id' property it must match the ID used in the URL.

Returning the updated drop doesn't seem useful (it should be
semantically equivalent to the uploaded data), so I've gone with '204
No Content' as the default success response.
public.json: Add 'DELETE /drop/{id}' to delete a drop

Returning the deleted drop doesn't seem useful (if you want more
information about it, ask before you delete it ;), so I've gone with
'204 No Content' as the default success response.
Merge branch 'drop-CrUD'

Create, update, and delete endpoints for drops.

* drop-CrUD:
  public.json: Add 'DELETE /drop/{id}' to delete a drop
  public.json: Add 'PUT /drop/{id}' to update a drop
  public.json: Add 'POST /drops' to create a new drop