Fix i18n for pluralized model names

On Rails 4.0, translating a model with count: 1.1 was returning the
singular version.
Improve Draper integration proof-of-concept [ci skip]
This "works", but we should not need to check if the collection is already decorated.

I found that the collection has already been decorated in lib/active_admin/resource_controller/data_access.rb; this still need some research...
Remove decorator duplication; improve collection decorator handling
Clean up collection decorator generated class caching
Refactor some names
Dynamic dependency checker initially for Draper

Noramally bundler would be used to ensure a specific version of a
dependency such as Draper, but we don't want to depend on Draper
at all.

Orginally written by @seanlinsley (see https://github.com/gregbell/active_admin/pull/2662)

Committed by @amiel
Use new Dependencies checker to branch on draper version
Also delegate group_values. Closes #2882
Fix for Draper < 1.0
Merge pull request #2662 from carnesmedia/draper-collections

Improve Draper integration
Allow index page_title to be set in controller
:scissors:
fix batch action / scope layout problem for other locales

Since CSS isn't yet advanced enough, we must resort to JS for this
layout to work with translations of different widths. :-/

This resolves #2888. Thanks to @philipgiuliani for pointing out the
problem.
Show page title can be set in controller
Merge pull request #2891 from quainjn/page_title_config

Allow index page_title to be set in controller
Add PLURAL_MANY_COUNT constant
Merge pull request #2791 from pcreux/fix-plural-i18n

Fix i18n for pluralized model names
cleanup
simplify load path code

this is less memory-efficient, but it's much easier to read :octopus: