Bumped up version of better_errors gem in Gemfile to improve code climate
add Coveralls to track test coverage
add coverage badge to README
added missing and fixed wrong german locales for comments
bugfix: apps no longer use the same namespace hash

This was a big issue when running tests, since this:
```ruby
app = ActiveAdmin::Application.new
app.namespace(:brand_new_ns)
```
Would end up creating `:brand_new_ns` in the main app, since our
settings module (which doesn't dup objects) was being used to create
the `namespaces` API.
cleanup
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!