Add a method to check in clean way if group exists

**What does this PR do?**
It adds a clean interface for checking if a group exists. That way you can have code like this:
```ruby
unless Flipper.group_exists?(:admins)
  group = Flipper.register('admins') { |actor| actor.admin? }
end
```

**Why is this PR needed?**
I didn't like the looks of having to put a resque block around `Flipper.register` to see if a group already existed. So this PR is not *needed* put it would be nice to have it in.
We are unregistering all groups before each spec so you shouldn't need these lines:

```
registry = Flipper::Registry.new
Flipper.groups = registry
```