Rake tasks to administer experiments

Adds rake tasks to administer experiments.

- Rake tasks are loaded using a Railtie, so it is part of the application's environment, and knows about the experiment definitions in the appliocation.
- Arguments are provided as environment variables
- Works with any storage model used (as long as the interface is implemented in the storage model).
- Con: much slower than manipulating Redis directly.

Example: 

``` shell
$ bundle exec rake experiments:assign_manually experiment=admin_pricing_reverse_experiment_nontrial subject=1234 group=reverse_order
$ bundle exec rake experiments:remove_assignment experiment=admin_pricing_reverse_experiment_nontrial subject=1234
$ bundle exec rake experiments:disqualify experiment=admin_pricing_reverse_experiment_nontrial subject=1234

$ bundle exec rake experiments:lookup_assignment experiment=admin_pricing_reverse_experiment_nontrial subject=1234
Subject 1234 is unqualified for experiment admin_pricing_reverse_experiment_nontrial
```