Relocate DslHelper from root namespace to under AASM namespace

`DslHelper` is a module for AASM and should not be located in the root
namespace.

lib/aasm/core/event.rb

@@ -2,7 +2,7 @@
 
 module AASM::Core
   class Event
-    include DslHelper
+    include AASM::DslHelper
 
     attr_reader :name, :state_machine, :options
 

lib/aasm/core/transition.rb

@@ -2,7 +2,7 @@
 
 module AASM::Core
   class Transition
-    include DslHelper
+    include AASM::DslHelper
 
     attr_reader :from, :to, :event, :opts, :failures
     alias_method :options, :opts

lib/aasm/dsl_helper.rb

@@ -1,4 +1,5 @@
-module DslHelper
+module AASM
+  module DslHelper
 
     class Proxy
       attr_accessor :options
@@ -27,4 +28,5 @@ module DslHelper
       proxy.options
     end
 
+  end
 end