Add :timestamp to publish payload

Now we can track when a particular result occurred.

lib/dat/science/experiment.rb

@@ -50,6 +50,8 @@ module Dat
       def run
         return run_control unless candidate? && enabled?
 
+        timestamp = Time.now
+
         if control_runs_first?
           control   = observe_control
           candidate = observe_candidate
@@ -59,6 +61,7 @@ module Dat
         end
 
         payload = {
+          :timestamp => timestamp,
           :candidate => candidate.payload,
           :control   => control.payload,
           :first     => control_runs_first? ? :control : :candidate

test/dat_science_experiment_test.rb

@@ -106,6 +106,8 @@ class DatScienceExperimentTest < MiniTest::Unit::TestCase
     assert_equal "foo", payload[:experiment]
     assert_equal :control, payload[:first]
 
+    assert (Time.now.to_i - payload[:timestamp].to_i) < 10
+
     assert payload[:control][:duration]
     assert_nil payload[:control][:exception]
     assert_equal :foo, payload[:control][:value]