fixes #84: allow group_by multiple fields

Fixes #84: allow group_by multiple fields.

To recover grouped values:

    search = MyClass.search :with => {:some_id => 85}, :group_by => "user_id, type";
    search.context[:panes] << ThinkingSphinx::Panes::AttributesPane
    search.each do |item|
      item.sphinx_attributes[‘count(*)’] # count
      item.sphinx_attributes[‘user_id’]
      item.sphinx_attributes[‘type’]
    end