Add support for unstarted experiments.

An experiment is considered unstarted if it has no start_date set. Such an
experiment will return nil for everyone's bucket, as if it had a population
filter that included no users.

The loader will leave an experiment unstarted if an experiment has an
"unstarted: true" attribute set.
Merge pull request #35 from howaboutwe/unstarted

Add support for unstarted experiments.