[digitalocean|compute] add created_at timestamp as attribute of server

* We receive this field from Digital Ocean, so all we need to do to use it is add it as an attribute and it just works

lib/fog/digitalocean/models/compute/server.rb

@@ -19,6 +19,7 @@ module Fog
         attribute :public_ip_address, :aliases => 'ip_address'
         attribute :private_ip_address
         attribute :backups_active
+        attribute :created_at
 
         attr_writer :ssh_keys
 

tests/digitalocean/models/compute/server_tests.rb

@@ -29,6 +29,7 @@ Shindo.tests("Fog::Compute[:digitalocean] | server model", ['digitalocean', 'com
         :flavor_id,
         :region_id,
         :image_id,
+        :created_at,
         :ssh_keys=
       ]
       tests("The server model should respond to") do