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)

This is already being set by `..._add_foreignkey...` migration. And again, we don't modify existing migrations. Please revert.