Add bootstrap 4 support

[pabse]

This adds bootstrap 4 support with proper class assignment and all possible configurations.
(Currently Bootstrap 4.0.0-beta)

TODO for the future: Fix specs
Also TODO: Add WIKI

Basic Usage (in haml):

= navbar expand_at: :sm, brand: image_tag('test.svg'), brand_url: root_path do
   = navbar_collapse id: 'navigation', class: 'justify-content-end' do
      = navbar_group do
        = navbar_item 'Dashboard', dashboard_path

Dropdowns should also work.

Fixes #12

[manuelmeurer]

💥 Fantastic, thank you! 😺