User can create a new trip
Add "TripMap" link to home page; Show user email
Associate Trip model and User model

* Add user_id to trip model
* Validates date, city and completed
* A user has many trips
* A trip belongs to a user
Add blank line in partial form
Create migration to add user id to Trip model

* Validates :completed in model level
Change completed column in seperate migration
Change column of timestamps in new migration
Update migration