Drops will now store images as Strings - the same way they are sent on the network.  There is no need to store a BufferedImage on the server because it is never accessed other than to send it to the client (which would require conversion back to a String).  Removed com.github.ayelix.deaddrop.server.Drop and renamed AbstractDrop to Drop, adding image String to the member list.
Added PickupServlet to handle /pickup requests (picking up drops).  Also minor documentation and debug output changes.