Device id is not a string anymore

* pushbullet.el: The new changes in API rolled out yesterday seem to
  have made device id as some sort of a hash. So removing the
  number-to-string conversion
Change alist to hash table

* pushbullet.el: Change the pb/device-id-list to pb/device-id-table
  which is a hash table. Thanks to `ht.el' we have a good and easy way
  of accessing key value pairs, avoids the lot of `cdr' `assoc' calls to
  obtain keys. Overall looks like a cleaner solution