Created sessions model, bare signin page
Handle invalid sign in with flash. Add tmux script

When a user presents invalid signin information, an
error flash appears and now correctly disappears when
a different page is clicked. Tests were added for this
behaivor. A Tmux script creates a rails dev environment.
Add remember_token to users table
Pass remember_token user_spec test.

Remember token is now created upon user creation and encrypted
before being stored in the database.
Add migration for User remember_token column
Add methods to sessions helper for signin/out
Add dynamic signin/signout link with dropdown.

Added a dropdown menu that is only visible to signed
in users as well as some other links to the header.
Add signout functionality