Give uploaded pastes "random IDs".

  This closes #1.

  We still have a hokey deletion-secret generation, which should
 now be updated, but the uploaded texts are now spider-free.

  Sample run:

     $ curl -H "Accept: application/json" -X POST --data "text=text+goes+here" http://markdownshare.com/create
     {"link":"http://markdownshare.com/view/f22692aa-2c6c-49f0-bcfb-2dff6642c5c6",
      "delete":"http://markdownshare.com/delete/1b1b12fe98dab12626b740e0ce1c24f6",
      "id":"f22692aa-2c6c-49f0-bcfb-2dff6642c5c6",
      "raw":"http://markdownshare.com/raw/f22692aa-2c6c-49f0-bcfb-2dff6642c5c6"}