added sessions controller
added :sessions resource to routes
removed .gitkeep from models folder
add float styling css classes
added default link styling
added focus styling to submit buttons
added sessions form
added login and logout routes
linked login page in header
set up sessions' controller actions
set up current_user method
added logout link if user is logged in
changed login failure flash message
added flash notice and alert messages to the view
change layout to flash messages
flash message styling
added js to auto hide flash messages and allow reopening of alert messages