Reap dead devices using timeouts. Previously, if a provider died without being able to clean up properly, the device would stay as a ghost.

lib/db/tables.js

@@ -1,3 +1,5 @@
+var r = require('rethinkdb')
+
 module.exports = {
   users: {
     primaryKey: 'email'
@@ -8,6 +10,7 @@ module.exports = {
       ownerEmail: function(device) {
         return device('owner')('email')
       }
+    , lastHeartbeatAt: null
     }
   }
 , logs: {