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)

ActiveRecord has the ability to remove_index on it's own through def change.  You don't need do to up/down for indexes