Add feature, turning off autoloading helpers

This is a ticket opened to request a Rails 2 behavior described in #3945 helper :all on by default.

The configuration should have the following settings:

1. (Default) Rails 3 behavior: all helpers defined in /app/helpers/ are loaded into every single controller, in an arbitrary order
   
2. Rails 2 behavior: only helper module associated with the controller is autoloaded
   
3. None: do not autoload any helpers

My proposal is:

# Rails 3 behavior
# No change

# Rails 2 behavior
self.autoload_only_controller_helpers = true

# No helper autoload, period
self.autoload_helpers = false

Let me know if anyone else finds this acceptable and we can get someone to submit this as a patch.