move setting inheritance into its own module

Also added missing tests for `ActiveAdmin::Settings`
Merge pull request #2313 from Daxter/bugfix/namespace-pollution

Bugfix: apps no longer use the same namespace hash
update README's Code Climate badge to show our grade
Mexican spanish messages added.

Created `es_MX.yml` file.
restored missing quotes
remove old DB migration for AA comments

Now when you create a new project with AA, only one DB migration is
used to create comments
bug fixes for AA Comments UI

+ fixes #367 regression by calling `to_s` on the namespace
+ fixes #2294 by applying queries to the passed object
+ disables eager load of polymorphic relationships for Rails 3.0 since they don't work
cleans up `active_admin_comments_for`
typo fix in CHANGELOG
make query method used for select filters more flexible

As per my discussion with @t-anjan in #1093
refactor: let `pluck` be easily testable
bugfix: always reorder when plucking a column

This is necessary to prevent PostgreSQL from erroring out when the SQL
SELECT query doesn't include the DB attributes specified in the ORDER
clause.

The added benefit is that any dropdowns created by this are
automatically ordered alphabetically!
make generated files singular instead of plural

Closes #2249
remove unnecessary backslashes in tests
update form builder to only use return val if it's a string

This closes #2192 by updating the fix for #887 to simply drop any
return value that isn't a string.
Merge 'unencapsulate-css-global-reset' of github.com/tinynumbers/active_admin

Conflicts:
	app/assets/stylesheets/active_admin/_base.css.scss
	lib/generators/active_admin/assets/templates/3.1/active_admin.css.scss
Default `rake` to `rake test`

The previous default was `rake parallel_tests`.

Parallel are awesome, but are failing on master right now (could be setup related, I don't know).

In any case, I'm not against parallel_tests, I'm just proposing that they not be the default.
Added a note about implementation details so if a page has a custom embedded form users have been at least warned of the consequences
update .gitignore with new RVM file names
Fix mass-assignment error caused by Devise 3.0.0

See plataformatec/devise#2515 for more information