Chen share trip

User can share and unshare a trip

![user can share and unshare a trip](https://cloud.githubusercontent.com/assets/3759510/5185571/631e0be4-748e-11e4-9c78-7b2e5071f6dd.gif)

I don't think you need `resource :trip_groups, only: [:create, :destroy]`

If you add those to the previous line, it's the same concept of posting to trip/trip_id/trip_groups/