Added Flipper.groups writer.

In case people want to have their own registry for some weird reason.

lib/flipper.rb

@@ -11,6 +11,10 @@ def groups
     @groups ||= Registry.new
   end
 
+  def groups=(registry)
+    @groups = registry
+  end
+
   def register(name, &block)
     group = Types::Group.new(name, &block)
     groups.add(group.name, group)

spec/flipper_spec.rb

@@ -7,6 +7,14 @@
     end
   end
 
+  describe ".groups=" do
+    it "sets groups registry" do
+      registry = Flipper::Registry.new
+      Flipper.groups = registry
+      Flipper.instance_variable_get("@groups").should eq(Flipper::Registry)
+    end
+  end
+
   describe ".register" do
     it "adds a group to the group_registry" do
       group = Flipper.register(:admins) { |actor| actor.admin? }