User can link to a login page from anywhere

  *1 feature test
  *added sessions routes
  *added link to layout
wrote a failing feature test for login
created sessions controller with a new action
created a login template
sessions controller spec
wrote a failing feature test for login
controller tests for sessions#create passing
controller test for invalid login passing
Feature tests for login passing

  *wrote current_user helper
  *added navbar logic for displaying login/logout
  *added logout to routes
wrote a failing feature test for log out
wrote failing controller tests for sessions#destroy
logout feature and controller specs passing
Merge pull request #4 from LightningLord/login_logout

Login logout