extracted a header element of page to a partial template.
layouted footer elements as sticky footer.
added SigninForm as model.
fixed the header navbar to use static-top class.
implemented the 'new' action in sessions_controller and wrote the specs.
added simple_form gem and run `rails g simple_form:install --bootstrap`.
implemented the signin form.
add factory_girl gem and configure to use it in spec_helper.
added find_user method to SigninForm and write its spec.
implemented the create action and wrote specs.
added a shared template to display flash error messages.
changed the flash messages life time.
added the error messages block to the signin form,
and adjusted the simple_form configurations in simple_form_bootstrap.rb.
added 'well' class to forms wrapper element.
wrote the sessions view spec.