Add sign up page

* Created user model and migration
* Added sign up view
* Added users controller
* Added link to layout
User sign up

* Added presence validation for user email
* Added users#create
* Show errors on users#new
Create sessions

* Implemented sessions#create
* Storing user info in cookies
Add current_user and signed_in? helper methods
Implement sign out

* Add destroy route for sessions
* Add sign out link
* Implment session#destroy