Chen create trip

https://trello.com/c/oqyHj7aE/22-as-a-user-i-can-create-a-new-trip

- current user can create a new trip
- a user has many trips
- a trip belongs to a user

![create a trip](https://cloud.githubusercontent.com/assets/3759510/5074718/0affe7fe-6e5b-11e4-9d84-311c801382f6.gif)

`update` is typically used for existing models. I would change this to:

``` ruby
@trip = current_user.trips.new(trip_params)
if @trip.save
```