implement token bucket rate limiting

db/migrate/20170106012138_create_token_buckets.rb

@@ -0,0 +1,10 @@
+class CreateTokenBuckets < ActiveRecord::Migration
+  def up
+  	execute "create unlogged table token_buckets (user_id integer, last_touched_at timestamp not null, token_count real not null)"
+  	add_index :token_buckets, :user_id, :unique => true
+  end
+
+  def down
+  	raise NotImplementedError
+  end
+end