Add Experiment#started_at that will store the 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. Memoizes the result, so we don't do tons of  Redis calls.
Add a test case to test it's intended usage.
Explicitly convert datetime to string when storing it in Redis.
Rename DummyStorage to MockStorage, and make MemoryStorage the default storage engine.
Make behaviour of starting an experiment more straightforward.
Use Time instead of DateTime, and convert to UTC to be sure.
Merge pull request #10 from Shopify/start_date

Add Experiment#started_at