implement get_image api call

lib/fog/joyent/requests/compute/get_image.rb

@@ -0,0 +1,29 @@
+module Fog
+  module Compute
+    class Joyent
+
+      class Mock
+        def get_image(id)
+          if ds = self.data[:datasets][id]
+            res = Excon::Response.new
+            res.status = 200
+            res.body = ds
+          else
+            raise Excon::Errors::NotFound
+          end
+        end
+      end
+
+      class Real
+        def get_image(id)
+          request(
+              :method => "GET",
+              :path => "/#{@joyent_username}/images/#{id}",
+              :expects => 200
+          )
+        end
+      end
+
+    end
+  end
+end