Fix minor bugs, potentially refactor helper.rb

The first commit adds an include_rails_helper option that controls whether or not helper is mixed into rails controllers by default. Given that some method names are pretty generic (i.e. reset!) that could lead to conflicts (like in our case). As a workaround I added `EncapsulatedHelper`, which only exposes `ab_test` and `ab_test_finished`. It's a bit of a hack and I wouldn't generally use it, but perhaps it makes sense to refactor the helper so it exposes fewer, more namespaced methods and can be mixed into arbitrary classes, not just controllers? We mix EncapsulatedHelpers into models, as well, and use a custom persistence adapter that can deal with the different context.

The other two commits fix an issue where the resettable property from Split.configuration.experiments wasn't picked up by normalize_experiments.

My apologies, I tried to submit this as separate pull requests but I didn't find that option on GitHub.

Great job on this gem, by the way! Thanks for taking the time to publish it!