add drag and drop for tasks

[RaimundSchramm]

• tests incomplete and suboptimal, mainly Controller and Integration

further reading:

• http://stackoverflow.com/questions/36214550/rspec-integration-testing-testing-how-many-anchor-tags
• http://blog.cynthiakiser.com/blog/2014/12/26/upgrading-from-rails-4-dot-1-8-to-4-dot-2-0/
• http://matthewlehner.net/rails-api-testing-guidelines/

capybara-and-sortable-related:

• http://makandracards.com/makandra/18023-capybara-trigger-requests-with-custom-request-method
• https://github.com/jnicklas/capybara/issues/222
• http://stackoverflow.com/questions/7796495/capybara-drag-drop-does-not-work
• http://heywill.com/blog/2012/12/15/using-capybara-to-drag-a-jquery-ui-sortable-onto-a-jquery-ui-droppable