Add group to scopes

[McRipper]

This change allow to define a group in the scopes so they can be grouped together with the relative label.

  scopes do
    Dossier.categories_for_select.each do |s|
      scope s[0], -> { Dossier.where(category: s[1]) }, group: 'Category'
    end
  end

[coveralls]

Coverage decreased (-0.03%) to 87.282% when pulling 11775bc on McRipper:master into 746f9ea on TrestleAdmin:master.

[spohlenz]

I've taken this idea and enhanced it with support for column vs row layouts, mobile support and some improved design. Here is how it looks on a client's project:

The usage is effectively the same as in your example. Provide the group option on a scope to enable grouping, and additional options can be specified on the scopes call.

# Default layout is :row (as shown in the screenshot) which is different from your initial PR
scopes layout: :columns do
  ...
end

# Disable grouping regardless of scope definitions
scopes group: false do
  ...
end

# Add custom classes to the scopes block
scopes class: "scope-tags" do
  ...
end

I'm aiming to release 0.9.5 tonight with this and other recent improvements and fixes.