Separate configuration from code

[phillbaker]

Consider that configuration should be separate from code, which might include features like:

• selectively enabling and disabling tests (without modifying code)
• default values for experiments (see apartmentlist/vanity#2)
• selectively enabling alternatives (this might be tied to #147)
• consider moving the path of experiments files from experiments to config/experiments to reinforce the configuration-nature of the experiments

The overall goal should be to ensure that enabling/updating an A/B test doesn't require a re-deployment. There is some limit to this: some A/B tests will require content/code changes that should be part of a deployment (like different views, different UX), however the goal should be to minimize these cases.