Adds extra attributes to the Image resource

* `min_disk_size`, `size_gigabytes` & `created_at`
* Adds them as attributes and properties, and 
updates the specs and fixtures.

lib/droplet_kit/mappings/image_mapping.rb

@@ -15,6 +15,10 @@ class ImageMapping
       property :public, scopes: [:read, :read_snapshot, :read_backup]
       property :regions, scopes: [:read, :read_snapshot, :read_backup]
       property :type, scopes: [:read, :read_snapshot, :read_backup]
+
+      property :min_disk_size, scopes: [:read, :read_snapshot, :read_backup]
+      property :created_at, scopes: [:read, :read_snapshot, :read_backup]
+      property :size_gigabytes, scopes: [:read, :read_snapshot, :read_backup]
     end
   end
-end
+end

lib/droplet_kit/models/image.rb

@@ -7,5 +7,8 @@ class Image < BaseModel
     attribute :public
     attribute :regions
     attribute :type
+    attribute :min_disk_size
+    attribute :size_gigabytes
+    attribute :created_at
   end
 end

spec/fixtures/images/find.json

@@ -9,6 +9,8 @@
       "region--1"
     ],
     "created_at": "2014-07-29T14:35:41Z",
-    "type": "snapshot"
+    "min_disk_size": 20,
+    "type": "snapshot",
+    "size_gigabytes":0.43
   }
-}
+}

spec/lib/droplet_kit/resources/image_resource_spec.rb

@@ -51,6 +51,10 @@
       expect(image.public).to eq(false)
       expect(image.regions).to eq(["region--1"])
       expect(image.type).to eq("snapshot")
+
+      expect(image.min_disk_size).to eq(20)
+      expect(image.size_gigabytes).to eq(0.43)
+      expect(image.created_at).to eq('2014-07-29T14:35:41Z')
     end
 
     it_behaves_like 'resource that handles common errors' do
@@ -86,4 +90,4 @@
       expect(image.id).to eq(146)
     end
   end
-end
+end