Add a  .rvmrc example

I don't add the final .rvmrc to the repository since it can change in
every machine (maybe, some developers don't want to use gemsets, use a
custom Ruby build, etc).

The .rvmrc.example is provided to be able to cp it.
Ignore /bin directory

The directory is generated with

  $ bundle install --binstubs

and it contains the executables located in the installed gems. With this,
we can run

  $ bin/rails whatever

Instead of

  $ bundle exec rails
Use the debugger gem

The ruby-debug19 is discontinued. The new gem is fully compatile with the
newest rubies, and it can be installed really fast with no tricks.

The gem is restricted to the development and test group, so it will not be
loaded in production mode (where there is no need to use a debugger).
Remove a .swp file
Use PostgreSQL instead of MySQL

All tests pass with no changes in PostgreSQL.
Refactor in db/seeds.rb

The db/seeds.rb has to be prepared to be loaded multiple times, so it has to
take care that there is no duplicated data.

The Group.find_or_create_by_name is used to create the group only when it is
not found. The instance is stored in the Groups hash to be used later.

We removed the use of `user.update_attribute`, since there is no reason to
store the instance (via `User.create`) and then modify the value. The new
code use `User.new.tap` to initialize everything and make only one INSERT
in the database.

There is no guarantee that the admin group has the id 1, so the code

  update_attribute(:group_id, 1)

is dangerous. If we reset the groups table, the new created admins group
will have a bigger value.

The example admin user is created only when there is no users, so we can
run `rake db:seed` in production with no risk. If we don't check if there are
any users in the database we could create a security hole.
Move #auto_pages to PagesHelper

The #auto_pages has be defined in the PagesHelper. It makes no sense to
be defined in the ApplicationController.
Remove #is_admin as a helper_method

This method can be used only as a before_filter, since it is using
some controller specific methods, like `redirect_to`.
Move recursive_childs to PagesHelper

This is related to the commit 7b3289cefcb57e77d
Refactor to ApplicationController#is_admin

We add a AdminUserIsRequired exception, so the helper is_admin can be used
at any point in the controller, not only in a before_filter.

In the previous version, when the client can not see the page, it was
redirected. This is very annoying, because the client loses the current URL.

The response sent a 403 (error) status , instead of a 302 (valid).

If the user has no logged, the application should show a form, and then
redirects to the original request.
Rewrite the page tree

* Use `content_tag`. A cleaner and safer way to generate HTML.
* Remove a lot of useless code.
* Vastly simplified version of the pages tree generator
Remove .sass-cache from repo
rails.png no longer used
Don't let the user change its own group

The user could remove himself from the admins group.
Add inherited_resources gem
Groups controller use the InheritedResources base
Pages controller now use the InheritedResources base

The `is_admin` before_filter has been added.
Merge remote-tracking branch 'origin/comments-on-12-users-groups' into 12-users-groups

Conflicts:
	.gitignore
	.rvmrc.example
	app/controllers/groups_controller.rb
PG changed to MySQL