Allow setting storage to :none for an experiment.

This will disable storing/caching subject assignments. This reduces overhead and required storage space for the experiment. In this case, you have to use a segmenter that will consistently segment an identifier to the same group. Also, you cannot assign or disqualify a subject by hand in this scenario.
Merge pull request #6 from Shopify/storage_none

Allow setting storage to :none for an experiment.