Add Experiment#started_at

It will store the start timestamp of the experiment.

It will be set to `DateTime.now` when being called first, and will return that value for every subsequent call using the storage backend (Redis). Memoizes the result, so we don't do tons of Redis calls.

### Example 

You can use this to qualify subjects based on the experiment start date, e.g.:

``` ruby
  qualify do |subject, context| 
    subject.created_at >= self.started_at
  end
```

### Questions 

- Do I need to keep timezones in mind for this?
- Should I use Time or DateTime

@camilo @peterjm @pseudomuto for review