Wrapper: add #feature_exists?

Easy way to check if a feature is already set in our featurelist or not.
MonkeyPatch: only write when feature is new.

Always writing is causing errors when you're using Rollout in a case
where you want certain parts to only read from slaves and other parts to
only write to your master.

As we're overwriting the standard `Rollout.active?` class, checking for
the `active` state in a read-only slave would cause a Redis error. Only
writing this feature when it doesn't exist resolves this issue.
MonkeyPatch: add test for conditional.

Add test to see that we'll only add a new feature if the feature doesn't
exist yet.
Merge pull request #1 from jelmersnoeck/jelmer/monkey-patch-fix

MonkeyPatch: don't write feature if it already exists.