Cleanup

I am working on a new version of this library to support a more advanced standard workflow. To start things off, I did some cleaning up of the codebase:

- Consistently use Experiments namespace.
- Removes Rails dependency, add an optional Railtie to replace it.
- Use bare bones MiniTest instead of ActiveSupport::TestCase & mocha.

Backwards compatibility note: everything should work as before, except the Experiment class is now namespaced as `Experiments::Experiment`. This should be an easy fix in Shopify.

@jamesmacaulay @kmcphillips @BenDoyle